test: add unit tests for external_lifecycle pure helpers

[northline-lab]

Summary

Add 48 dedicated unit tests for pure helper functions in contribarena.engine.external_lifecycle that previously had no dedicated test coverage.

Test Coverage

• ParseTest (6 tests) — ISO datetime parsing with Z suffix, UTC offset, positive/negative offsets, naive datetime assumes UTC, microseconds
• IsoTest (4 tests) — UTC-aware, naive assumes UTC, positive/negative offset conversion to UTC
• GithubStateTest (5 tests) — None/open, not-ok defaults to open, merged, closed not merged, open
• HasRequestedChangesTest (6 tests) — changes_requested detection, case-insensitive, approved, empty list, multiple reviews, missing state attribute
• HasRejectionSignalTest (6 tests) — rejection/opt_out/anti_ai_or_bot detection, positive exclusion, empty list, mixed signals
• SummaryForStatusTest (7 tests) — all 6 known statuses plus unknown fallback
• ClassifyStatusTest (10 tests) — full priority chain: not-ok → merged → rejection → closed → changes → CI failure → tracking, plus pr=None variants and merged-overrides-changes
• SignalsFromStatusTest (4 tests) — non-needs_response returns empty, needs_response without changes returns empty, creates process_feedback signal, deduplicates existing signals

Why This Matters

These helpers are central to the external PR lifecycle tracking contract:

• _parse/_iso handle GitHub's various timestamp formats consistently
• _classify_status implements the priority-ordered lifecycle classification
• _github_state maps PR API results to lifecycle states
• _signals_from_status generates maintainer signals with deduplication

Verification

pytest -q tests/unit/test_external_lifecycle.py -v
# 48 passed in 0.30s

ruff check tests/unit/test_external_lifecycle.py
# All checks passed!

Risk

Low — test-only addition. No production code is modified.

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This PR was created autonomously by an AI agent participating in ContribArena's evaluation framework.