Scope fallback pongs on 2.1

[goldyfruit]

Fixes #807.

Backports the scoped fallback pong collection fix onto the 2.1 line used by older runtimes.

Each fallback ping gets a fallback_request_id, and the collector ignores pongs for a different request. Legacy pongs without an id still work. The fallback registry is also read from a locked snapshot during each query, so register/deregister events cannot mutate the handler map while a request is collecting replies.

Checked with:

PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 python -m pytest -q test/unittests/test_fallback_service.py
python -m ruff check ovos_core/intent_services/fallback_service.py test/unittests/test_fallback_service.py

[coderabbitai]

📝 Walkthrough

Walkthrough

FallbackService now tags fallback pings with per-request IDs, filters pong acknowledgements by matching ID, uses a per-call response event, and reads fallback registry state through a locked snapshot helper. Tests cover matching, mismatching, and snapshot behavior.

Changes

Per-request fallback ack scoping

Layer / File(s)	Summary
Request id matching and registry snapshotting ovos_core/intent_services/fallback_service.py	Adds per-request fallback_request_id generation and propagation, ignores pong acks with non-matching IDs, switches to a local response_event, and protects fallback registry access with a lock-backed snapshot helper.
Threaded tests for pong matching and snapshot isolation test/unittests/test_fallback_service.py	Updates the test helper for the registry lock and adds tests for mismatched and matching pong IDs, context-based IDs, empty-string IDs, and isolated registry snapshots after mutation.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Sequence Diagram(s)

sequenceDiagram
  participant FallbackService
  participant SkillBus
  participant FallbackSkill

  FallbackService->>FallbackService: generate/reuse fallback_request_id
  FallbackService->>SkillBus: emit ping with request id in data/context
  SkillBus->>FallbackSkill: deliver ping
  FallbackSkill-->>SkillBus: pong with ack_request_id
  SkillBus-->>FallbackService: pong received
  alt ack_request_id matches
    FallbackService->>FallbackService: response_event.set()
  else ack_request_id mismatched
    FallbackService->>FallbackService: ignore ack
  end
  FallbackService->>FallbackService: response_event.wait()/clear()

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Possibly related issues

• Echo fallback request ids in pongs ovos-workshop#464 — Matches the request-id echoing and matching behavior implemented for fallback ping/pong handling.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The code stamps fallback pings with request IDs, filters mismatched pongs, and preserves legacy pongs without IDs.
Out of Scope Changes check	✅ Passed	The lock and snapshot changes are supporting concurrency fixes and stay within the scoped fallback collection work.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly reflects the main change: scoping fallback pong handling for the 2.1 backport.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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