remove unreachable!(), improve panic messages, fix tautological test

[anaslimem]

Summary

• Remove unreachable!() from retry loop (mcp_bridge.rs): Refactored connect_with_retry to use a last_error accumulator, eliminating the unreachable!() macro and replacing it with a real error return. The logic is now self-evident without tracing the loop.
• Improve mutex panic messages (trace-viewer/lib.rs): Changed four bare .unwrap() calls on Mutex::lock() to .expect("events mutex poisoned"). Same behaviour, but a crash now tells you exactly what failed instead of a generic Rust internal message.
• Fix example placeholder string (simple_agent.rs): Replaced the "TODO" literal in the dispatch error handler with "unhandled message" a string safe to ship and accurate for users who copy this example.
• assert! → debug_assert! for internal invariant (conductor.rs): assert!(self.proxies.is_empty()) was firing in release builds. Changed to debug_assert! since this guards an internal precondition, not user input catches bugs in development without hard-crashing production.
• Fix tautological test assertion (jsonrpc_error_handling.rs): assert!(result.is_ok() || result.is_err()) is always true and validates nothing. Replaced withassert!(result.is_ok(), ...) so when issue docs: add missing READMEs for sacp-tee, sacp-test, and yopo crates #64 is fixed and the #[ignore] is removed, the test actually guards the intended behaviour.