fix: add BACKEND_TIMEOUT error code with proper error handling

[refcell]

Summary

• Adds BACKEND_TIMEOUT error code constant (-32011) to the error_codes module
• Adds explicit BackendTimeout match arm in to_error_payload() instead of falling through to internal_error()
• Uses inline format args for consistency with modern Rust style
• Adds test coverage for BackendTimeout error code and payload message content

Changes

Previously, BackendTimeout errors fell through to internal_error() in to_error_payload(), returning a generic -32603 code. Now it returns a specific -32011 timeout error code with a descriptive message containing the backend name.

This improves client-side error handling by allowing callers to distinguish timeout errors from other internal errors.

Testing

• Added test case for BackendTimeout in test_error_payload_code
• Added test_backend_timeout_payload_contains_name test to verify message content
• All 24 tests pass

[refcell]

Review: ✅ LGTM

Clean implementation:

• Error code -32011 follows existing pattern with BACKEND_OFFLINE at -32010
• Explicit BackendTimeout match arm is cleaner than falling through to internal_error()
• Good test coverage added
• Inline format args are consistent with modern Rust style

Verified:

• ✅ Clippy clean
• ✅ All tests pass