fix(sse): widen compression worker gate to accept structured-clone-safe values (#13154)

[diegosouzapw]

Closes #13154

Root cause

isStrictlySerializable in open-sse/services/compression/compressionWorkerProtocol.ts did
if (typeof value !== "object") return false; before anything else, so any undefined value
anywhere in the tree rejected the whole object — and any non-plain object (Date/Map/Set/
RegExp) failed isPlainObject and was rejected too, even though structuredClone (what
worker.postMessage actually uses) accepts all of these natively.

strategySelector.ts's runCompressionAsync always builds the 9-key workerOptions object
with every key explicitly present (model, supportsVision, providerTransport, provider,
imageTransportFidelity, sourceFormat, targetFormat, compressionStage, config), so
provider: undefined alone (the common case when no provider has been resolved yet) rejected
almost every real stacked/rtk/standard compression call, forcing the heavy caveman/rtk
pipeline onto the main event loop instead of a worker thread.

Fix

• isStrictlySerializable: accept undefined immediately (it round-trips through
  structuredClone as an absent/undefined key), and accept Date/Map/Set/RegExp (copied
  natively by structuredClone, not walked recursively) before the isPlainObject rejection.
  Functions and Symbols are still rejected; the existing path-based cycle detection (seen
  add/delete, fix(compression): track only the recursion path in isStrictlySerializable (#13154) #13423) is untouched.

• Companion fix (found while validating this change, same PR): compressionWorker.ts's
  stacked-mode branch called applyStackedCompression directly on the raw job body, skipping
  the adaptBodyForCompression/restore step that the sync in-process path
  (strategySelector.ts's runCompression) always applies for Responses input[] and Kiro
  conversationState envelopes. Because the over-strict gate above meant essentially no real
  stacked call ever reached the worker, this branch was dead code — discovered when the
  existing preserves Responses bodies and hard-budget results test started failing once the
  gate widening actually routed that call to the worker for the first time (worker output
  diverged from the sync path: off-by-one token count, missing hard-budget validation warning).
  Without this companion fix, widening the gate alone would have shipped a live regression
  (miscompressed Responses/Kiro bodies whenever routed to the worker). Fixed by mirroring the
  same adapt/restore wrapping in the worker's stacked branch.

Regression test

New file tests/unit/compression-worker-gate-13154-repro.test.ts (mirrors the plan-file's
proven repro exactly). Before the fix:

✖ EXPECTED (currently RED): the gate should accept that same structured-cloneable shape
  AssertionError: false !== true
✖ EXPECTED (currently RED): realistic runCompressionAsync-shaped call (only `provider` unset) should be eligible
  AssertionError: false !== true
ℹ tests 3
ℹ pass 1
ℹ fail 2

After the fix:

✔ structuredClone accepts a workerOptions shape with an explicit `undefined` key
✔ the gate should accept that same structured-cloneable shape
✔ realistic runCompressionAsync-shaped call (only `provider` unset) should be eligible
ℹ tests 3
ℹ pass 3
ℹ fail 0

Gates run

• node --import tsx/esm --test tests/unit/compression-worker-gate-13154-repro.test.ts → 3/3 pass
• node --import tsx/esm --test tests/unit/compression/compression-worker.test.ts → 13/13 pass (full suite, including the companion-fix regression)
• npm run typecheck:core → exit 0
• node scripts/check/check-open-sse-typecheck.mjs → openSseTypecheckErrors=0, OK
• npx eslint --suppressions-location config/quality/eslint-suppressions.json <changed files> → 0 errors
• node scripts/check/check-file-size.mjs → OK, no frozen file over its baseline
• node scripts/check/check-changelog-integrity.mjs → OK
• npx eslint --no-config-lookup --config eslint.complexity.config.mjs (compressionWorker.ts, scoped) → 0 violations
• npx eslint --no-config-lookup --config eslint.sonarjs.config.mjs (compressionWorker.ts, scoped) → 0 violations
• Full-repo check-complexity.mjs / check-cognitive-complexity.mjs ratchets (pre-companion-fix baseline) → both OK, within baseline

Existing tests aligned

tests/unit/compression/compression-worker.test.ts:

• Split rejects functions, symbols, classes, special objects, cycles, and non-finite numbers
  into rejects functions, symbols, cycles, and non-finite numbers (kept) plus two new tests
  asserting Date/Map/Set/RegExp and undefined are now accepted — this is alignment to
  the corrected contract (these values are structured-clone-safe), not a weakening: the cycle,
  function, Symbol and non-finite-number rejections are all still asserted.
• Added a new regression test mirroring strategySelector.ts's exact 9-key workerOptions
  shape with provider unset, asserting isCompressionWorkerEligible(...) === true.
• preserves Responses bodies and hard-budget results needed no edit — it started passing
  again once the companion fix above was added (see Root cause / Fix for the investigation).

Plan-file: _tasks/pipeline/bugs/2-implementing/13154-fix-backend-compression-worker-gate-rejects-structured-cloneab.plan.md