test: add unit tests for audit-07 coverage gaps
Add 81 new tests addressing audit-07 findings across 4 areas:
Provider factory resolution (42 tests):
- Cover all 25+ untested providers and aliases in factory
- Test openrouter, gemini, bedrock, copilot, china region, local,
cloud AI, and custom endpoint providers
Config schema boundaries (26 tests):
- Invalid value fail-fast (wrong types, overflow port)
- Gateway, security, autonomy config defaults and roundtrips
- Backward compatibility (unknown keys, partial sections)
- Nested optional section defaults
Gateway rate limiter boundaries (8 tests):
- Window expiry and re-allow after cooldown
- Independent key tracking
- Exact max_keys boundary eviction
- Pair vs webhook independence
- Concurrent access thread safety
- Rapid burst then cooldown pattern
Tool error paths (5 tests):
- Null byte in path rejection for file_read and file_edit
- Shell nonexistent command, stderr capture, action budget exhaustion
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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