fix(gateway): fail closed on compression state probe errors

[necoweb3]

Summary

Fail closed when the gateway cannot verify whether context compression is currently in flight for a busy session.

Problem

GatewayRunner._session_has_compression_in_flight() protects the #56391 path: when a user sends a follow-up while context compression is rotating the session, busy_input_mode='interrupt' must be demoted to queue semantics. Otherwise the gateway can interrupt/start work against the pre-rotation parent session and fork orphaned compression siblings.

The helper currently returns False for all exceptions while checking the session routing entry or reading get_compression_lock_holder(). That makes transient store/SQLite probe failures fail open: the gateway treats compression as absent and allows the interrupt path to proceed.

Fix

Split structural/version-skew absence from real probe failures:

• AttributeError / TypeError still return False, preserving the fix(compression): fail open when lock subsystem is missing (version skew) #34475 compatibility behavior for older/missing lock subsystems.
• Unexpected store or DB probe errors return True, treating compression as active so the busy handler queues the follow-up instead of interrupting the parent session.

Tests

python -m pytest tests/gateway/test_compression_in_flight_check.py tests/gateway/test_compression_interrupt_demotion_56391.py tests/gateway/test_priority_path_compression_demotion_56391.py -q --timeout-method=thread
18 passed