Fix: status could not be serialised when the probe SUCCEEDED

The probe returned `error: undefined` on a dimension match. `undefined` is not a
JSON value -- JSON.stringify silently drops it from an object -- so the shape
looked harmless and was not: the harness enforces the declared output schema
against the canonical value before materialising it, and an explicit undefined
property fails that check. A perfectly healthy status call could not be returned
at all.

Fixed at the boundary rather than at each site. Every return now funnels through
`toJsonValue`, which drops undefined properties exactly as JSON.stringify would,
maps non-finite numbers to null (same reasoning: JSON.stringify(NaN) is "null"),
turns an undefined array slot into null rather than shifting indices, and refuses
a circular value instead of recursing forever. `update` was carrying
`errorCode: undefined` and `ensureSlopo` a `detail: undefined` on the same path,
so a per-site fix would have left two more of these waiting.

The probe also now omits `error` rather than assigning undefined, since it is a
plausible thing for another consumer to read.

Four regression tests, and they were checked against a negative control: with the
sanitiser reverted and the undefined restored, exactly the "probe SUCCEEDS" test
fails with "does not survive a JSON round trip" and the others still pass, which
is what makes it a test of this bug rather than of the JSON round trip in
general.

test/plugin.test.js

@@ -76,7 +76,7 @@ const shell = {
  * `jobs` is deliberately absent, which forces the tool down its foreground path
  * so the assertions can see a result rather than a job handle.
  */
-function harness(overrides = {}, { withSettings = true, withJobs = false, sandboxMode, sandboxPolicyResolver } = {}) {
+function harness(overrides = {}, { withSettings = true, withJobs = false, withShell = true, sandboxMode, sandboxPolicyResolver } = {}) {
   const config = { ...FULL_CONFIG, ...overrides }
   // A per-harness shell, so a test can vary sandboxMode and every resolved
   // request is recorded for assertion.
@@ -101,7 +101,7 @@ function harness(overrides = {}, { withSettings = true, withJobs = false, sandbo
   const ctx = {
     logger: () => ({ info() {}, warn() {}, debug() {}, error() {} }),
     get(name) {
-      if (name === 'shell') return scopedShell
+      if (name === 'shell') return withShell ? scopedShell : undefined
       if (name === 'jobs') return withJobs ? state.jobs : undefined
       if (name === 'sandboxPolicy') return sandboxPolicyResolver
       return undefined
@@ -432,3 +432,104 @@ test('configure never touches the shell, and status only runs a read-only versio
     assert.match(request.command, /--version/, 'status may only run a read-only version check')
   }
 })
+
+/**
+ * Walk a returned value and fail on anything that is not lossless JSON.
+ *
+ * `assert.deepStrictEqual` against a JSON round trip catches an `undefined`
+ * property (Node distinguishes `{a:1,b:undefined}` from `{a:1}`), but the walk
+ * names the offending path, which matters when the value is a deep diagnostic.
+ */
+function assertLossless(label, value) {
+  assert.deepEqual(value, JSON.parse(JSON.stringify(value)), `${label}: does not survive a JSON round trip`)
+  const walk = (node, path) => {
+    if (node === null || typeof node !== 'object') return
+    if (Array.isArray(node)) {
+      node.forEach((item, index) => walk(item, `${path}[${index}]`))
+      return
+    }
+    for (const [key, entry] of Object.entries(node)) {
+      assert.notEqual(entry, undefined, `${label}: ${path}.${key} is undefined, which is not lossless JSON`)
+      walk(entry, `${path}.${key}`)
+    }
+  }
+  walk(value, label)
+}
+
+test('status returns lossless JSON when the endpoint probe SUCCEEDS', async () => {
+  // The regression: the probe used to return `error: undefined` on a match, and
+  // an explicit undefined property failed the harness's output validation, so a
+  // perfectly healthy status call could not be serialised at all.
+  const { createServer } = await import('node:http')
+  const server = createServer((request, response) => {
+    response.writeHead(200, { 'content-type': 'application/json' })
+    response.end(JSON.stringify({ data: [{ embedding: new Array(768).fill(0.01) }] }))
+  })
+  await new Promise((resolve) => server.listen(0, '127.0.0.1', resolve))
+  const { port } = server.address()
+  try {
+    const { state } = harness({
+      embeddingModel: 'ollama/example:latest',
+      embeddingDimensions: 768,
+      embeddingApiBase: `http://127.0.0.1:${port}`,
+    })
+    const value = await state.tools.get('slopo').execute({ action: 'status' }, execContext('/tmp'))
+    assertLossless('status', value)
+    assert.equal(value.ok, true)
+    assert.equal(value.details.embeddingEndpoint.ok, true)
+    assert.equal(value.details.embeddingEndpoint.dimensions, 768)
+    assert.equal(Object.hasOwn(value.details.embeddingEndpoint, 'error'), false)
+  } finally {
+    await new Promise((resolve) => server.close(resolve))
+  }
+})
+
+test('status returns lossless JSON when the endpoint probe FAILS', async () => {
+  const { state } = harness({
+    embeddingModel: 'ollama/example:latest',
+    embeddingDimensions: 768,
+    // Nothing is listening on this port, so the fetch rejects.
+    embeddingApiBase: 'http://127.0.0.1:1',
+  })
+  const value = await state.tools.get('slopo').execute({ action: 'status' }, execContext('/tmp'))
+  assertLossless('status (unreachable)', value)
+  assert.equal(value.ok, false)
+  assert.ok(value.details.embeddingEndpoint.error.length > 0)
+})
+
+test('every action that returns without executing anything is lossless JSON', async () => {
+  const cases = [
+    [{ action: 'status' }, 'status (unconfigured)'],
+    [{ action: 'configure', config: { embeddingModel: 'x', nope: 1 } }, 'configure (unknown setting)'],
+    [{ action: 'configure', config: { embeddingModel: 'x' } }, 'configure (ok)'],
+    [{ action: 'configure', config: 'not-an-object' }, 'configure (bad patch)'],
+    [{ action: 'review' }, 'review (unconfigured)'],
+  ]
+  for (const [args, label] of cases) {
+    const { state } = harness()
+    const value = await state.tools.get('slopo').execute(args, execContext('/tmp'))
+    assertLossless(label, value)
+  }
+})
+
+test('an action refused before it runs is lossless JSON', async () => {
+  const configured = {
+    embeddingModel: 'ollama/example:latest',
+    embeddingDimensions: 768,
+    embeddingApiKey: 'secret',
+    embeddingApiBase: 'http://127.0.0.1:11434',
+  }
+
+  const noShell = harness(configured, { withShell: false })
+  const refused = await noShell.state.tools.get('slopo').execute({ action: 'review' }, execContext('/tmp'))
+  assertLossless('review (no shell)', refused)
+  assert.equal(refused.errorCode, 'NO_SHELL')
+
+  const outside = harness(configured, {
+    sandboxMode: 'workspace-write',
+    sandboxPolicyResolver: { resolve: () => ({ mode: 'workspace-write', workspaceRoot: '/some/workspace' }) },
+  })
+  const value = await outside.state.tools.get('slopo').execute({ action: 'review' }, execContext('/some/workspace'))
+  assertLossless('review (state outside sandbox)', value)
+  assert.equal(value.errorCode, 'STATE_OUTSIDE_SANDBOX')
+})