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.

lib/probe.js

@@ -143,7 +143,7 @@ export async function probeEmbeddingEndpoint({
     }
     const actual = vector.length
     const expected = Number(dimensions)
-    return {
+    const result = {
       checked: true,
       reachable: true,
       ok: actual === expected,
@@ -151,11 +151,14 @@ export async function probeEmbeddingEndpoint({
       sentModel,
       dimensions: actual,
       expected,
-      error:
-        actual === expected
-          ? undefined
-          : `the model returns ${actual} dimensions but embeddingDimensions is ${expected}`,
     }
+    // Omitted rather than assigned undefined: an explicit `undefined` property
+    // is not lossless JSON and fails the tool's output validation before the
+    // result is ever materialised.
+    if (actual !== expected) {
+      result.error = `the model returns ${actual} dimensions but embeddingDimensions is ${expected}`
+    }
+    return result
   } catch (error) {
     const aborted = error?.name === 'AbortError'
     return {