fix(compact): sanitize plaintext compaction replays

[Komzpa]

Summary

• strip poisoned local compact fallback messages together with their following compact output item
• rewrite plaintext replayed compaction summaries into inert assistant text so old local compact summaries do not go back upstream as compaction items
• preserve provider-looking encrypted compaction state (gAAAA...) unchanged on both normal /responses continuation and compact requests

This replaces the earlier too-broad encrypted-state rewrite: valid provider compact state is authoritative continuity and must stay opaque, not become [compact state] [unverified compact state omitted].

Live failure and follow-up

The original invalid-encrypted-content failures were caused by replaying local/plaintext compact fallback state as a provider compaction item. A later live post-compact hang showed the opposite hazard too: replacing valid provider encrypted compact state broke continuation context. This PR now keeps the two cases separate.

Testing

• uv run pytest tests/unit/test_openai_requests.py::test_compact_rewrites_plaintext_compaction_replay_without_encrypted_content tests/unit/test_openai_requests.py::test_compact_preserves_provider_encrypted_compaction_replay tests/unit/test_openai_requests.py::test_responses_preserves_provider_encrypted_compaction_replay tests/unit/test_openai_requests.py::test_compact_strips_poisoned_local_compact_fallback_items tests/unit/test_openai_requests.py::test_responses_strips_poisoned_local_compact_fallback_items -> 5 passed
• uv run pytest tests/unit/test_openai_requests.py -> 172 passed
• PR scope gate: git diff --name-only origin/main...HEAD contains only app/core/openai/requests.py and tests/unit/test_openai_requests.py

[chatgpt-codex-connector]

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