m12-pr15: DLQ contract pin (tests/queue/test_dlq.py)

[BalaShankar9]

Summary

Adds backend/tests/queue/test_dlq.py — 8 tests that pin the DLQ
behaviour the operator runbook (scripts/ops/dlq_replay.py) and the
queue_dlq_total Prometheus snapshot rely on.

No production code changed.

Why

The existing backend/tests/core/events/test_stream_consumer.py covers
exactly one DLQ path (test_dlq_after_max_deliveries_via_reclaim).
That test asserts behavioural shape but not the contract that
downstream consumers (operator runbook, replay tool, metrics snapshot)
depend on.

If a future refactor drops a field, swaps the xadd/xack order, or
collapses the two DLQ branches, the queue keeps working but the runbook
silently breaks.

What's pinned

#	Test	Invariant
1	test_dlq_entry_has_full_5_field_schema	exact 5 keys: consumer, source_stream, source_msg_id, reason, event
2	test_dlq_xadd_happens_before_source_xack	crash-safety: a crash between the two leaves a duplicate (recoverable) not a drop
3	test_dlq_reason_truncated_to_500_chars	bounded log + metric label cardinality
4	test_dlq_increments_queue_dlq_metric	queue_dlq_total Prometheus snapshot counter wires up
5	test_dlq_inline_when_delivery_attempt_already_exceeds	delivery_attempt > max_deliveries branch is distinct from "handler raised on final attempt"
6	test_dlq_decodes_bytes_fields_losslessly	real redis-py returns bytes; the round-trip must produce a JSON-decodable string with the original event_id
7	test_dlq_entry_replays_via_dlq_replay_tool	scripts/ops/dlq_replay.py::_build_replay_payload accepts the DLQ entry as-is
8	test_no_dlq_when_handler_eventually_succeeds	negative control: success path produces no DLQ entry, source ack present

Local verification

cd backend && pytest tests/queue/test_dlq.py -v
# 8 passed in 0.11s

Stacked on

PR #32 (m12-pr14-requirements-lockfile). Base of this PR is
m12-pr14-requirements-lockfile; merging that first will collapse
the diff to the m12-pr15 surface only.

Files

• backend/tests/queue/__init__.py (new, empty)
• backend/tests/queue/test_dlq.py (new, 8 tests, ~280 lines)
• context/TESTING_CONTEXT.md — m12-pr15 row in "shipped"
• context/CHANGELOG_INTELLIGENCE.md — m12-pr15 row

[Copilot]

Pull request overview

Adds a dedicated DLQ contract test module to pin the Redis Streams consumer’s DLQ invariants relied on by operator tooling and metrics, plus updates the /context ledger to reflect the shipped work. No production code paths are modified.

Changes:

• Added backend/tests/queue/test_dlq.py with 8 async tests that lock down DLQ schema, ordering, truncation, metrics, bytes decoding, replay-tool compatibility, and negative-control behavior.
• Added backend/tests/queue/__init__.py to introduce the tests.queue package.
• Updated context/TESTING_CONTEXT.md and context/CHANGELOG_INTELLIGENCE.md to move the DLQ test entry to “Shipped (m12-pr15)” and add the PR ledger row.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 1 comment.

File	Description
context/TESTING_CONTEXT.md	Moves DLQ contract tests from “Planned” to “Shipped” with a brief invariant summary.
context/CHANGELOG_INTELLIGENCE.md	Adds the m12-pr15 ledger row and corrects the m12-pr14 PR reference to #32.
backend/tests/queue/test_dlq.py	New DLQ contract test suite covering schema, crash-safety ordering, truncation, metrics, branch distinction, bytes decoding, replay-tool compatibility, and success-path non-DLQ.
backend/tests/queue/init.py	New (empty) package initializer for queue tests.

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