[Fix] Add GatewayDispatcher error-dedup and lifecycle-buffer tests

[YuriNachos]

Summary

Adds the first tests for the GatewayDispatcher error-display paths called out in
#227: isSameFailure (runId and code+source branches), shouldDisplayError
(incl. the DEDUP_FALLBACK_WINDOW_MS fallback), and the lifecycleErrorBuffer
2s buffer-and-clear behavior. No production code changes — test-only.

Type of change

• [Fix] bug fix (test coverage for an existing untested error path; no behavior change)

Why is this needed?

The error-dedup and lifecycle-buffer logic in packages/core/src/services/gateway-dispatcher.ts
had zero tests (#227). A silent regression there means users either get spammed
with duplicate errors or see no errors at all. This locks down the behaviors that
decide whether an error reaches the user.

What changed?

• New packages/core/test/gateway-dispatcher.test.ts (vitest). It drives the
  public createGatewayDispatcher via a harness that mirrors
  packages/desktop/test/gateway-dispatcher-subagent.test.ts and covers:
  1. isSameFailure suppresses a repeat with same taskId+code+source, and
     displays when the code differs;
  2. isSameFailure suppresses a repeat with same taskId+runId, and displays
     when the runId differs;
  3. shouldDisplayError shows the first error for a task, suppresses an
     identical repeat;
  4. the DEDUP_FALLBACK_WINDOW_MS (2000 ms) fallback suppresses a same
     rawMessage+source repeat inside the window and re-displays after it
     elapses (vi.useFakeTimers + advanceTimersByTime);
  5. the agent lifecycle error is buffered for 2000 ms then released as a system
     message, and is cleared when a chat error for the same task arrives first.

Architecture impact

• Owning layer: core (test-only; no production code touched)
• Cross-layer impact: none
• Invariants touched: none — no behavior, signature, or visibility change
• Why those invariants remain protected: only a new test file under
  packages/core/test/; dependency direction (shared ← core) and layer
  ownership are unchanged.

Linked issues

Fixes #227

Validation

• pnpm lint (ran pnpm eslint on the new file — clean; full repo lint not run)
• pnpm test (ran pnpm --filter @clawwork/core test — 117/117 green, incl. the 8 new tests)
• pnpm build
• pnpm check:ui-contract
• Manual smoke test
• Not run (full pnpm check not run: it stops at the repo's pre-existing
  format:check baseline red (~539 files) that reproduces on untouched main.
  The new file passes prettier --check, eslint, and the core test typecheck
  (tsc -p tsconfig.test.json — exit 0).)

$ pnpm --filter @clawwork/core test
 Test Files  7 passed (7)
      Tests  117 passed (117)

$ pnpm prettier --check packages/core/test/gateway-dispatcher.test.ts
All matched files use Prettier code style!

$ pnpm --filter @clawwork/core exec tsc -p tsconfig.test.json --noEmit   # clean
$ pnpm eslint packages/core/test/gateway-dispatcher.test.ts              # clean

[github-actions]

Hi @YuriNachos,
Thanks for your pull request!
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[YuriNachos]

/auto-cc