Fix #39704: Guard session transcript rewrite on successful rotation

[iamlukethedev]

Fixes #39704: Session Hygiene compression overwrites original messages when _session_db is None, permanently deleting 200+ messages.

Problem

• Gateway Session Hygiene compression checks if session rotation succeeded by comparing old/new session IDs
• When _session_db is None, compress_context() skips rotation, leaving session ID unchanged
• BUT rewrite_transcript() was called UNCONDITIONALLY on the original session
• Result: 200+ original messages deleted and replaced with ~7 compressed messages (irreversible data loss)

Root Cause

• Line 9314 comment said 'Write to NEW session so old transcript stays intact'
• But line 9327 rewrite_transcript() was unconditional while rotation was conditional
• When rotation failed, transcript was still overwritten

Solution

• Move rewrite_transcript() INSIDE the if-block confirming successful rotation
• Only rewrite when _hyg_new_sid != session_entry.session_id
• When rotation fails, keep compressed in memory only, don't persist to DB
• Add warning when rotation fails

Tests

• 4 new tests verify rotation guard works correctly
• All 152 compression tests pass
• No regressions in agent context compression

[teknium1]

Automated hermes-sweeper review: this PR's fix is now implemented on current main.

Evidence:

• gateway/run.py:9274 computes the hygiene compression session id and gateway/run.py:9297 calls rewrite_transcript() only when rotation occurred or in-place compaction succeeded; the no-rotation branch preserves the original transcript and logs a warning.
• tests/gateway/test_session_hygiene.py:399 covers the no-rotation regression and asserts rewrite_transcript is not called.
• The implementing commit is 4c349e85f8e88acb5f970705a3fd16a469d76d25 (fix(gateway): preserve transcript when hygiene auto-compress can't rotate), which is present on current main.

Thanks for the report and patch — the current mainline fix also accounts for the in-place compaction success path while preserving the same data-loss guard.