fix(rust): respond when request handlers panic

[hogeheer499-commits]

Fixes #2053.

The Rust session event loop spawns each inbound JSON-RPC request handler independently. If one of those futures panics, Tokio isolates the panic to that task, but the request receives no response and the caller waits until its own timeout.

This catches unwinds at the request-dispatch boundary and returns JSON-RPC internal error (-32603) for the affected request. The panic payload is not exposed. It also updates the stop_event_loop documentation and tests to reflect the current lifecycle: independently spawned handlers may finish after the parent event loop exits.

Regression coverage verifies both the internal-error response and post-loop handler completion.

Tests:

• cargo test --no-default-features --features test-support --lib (182 passed)
• Non-live Rust integration targets, including session_test (144 passed, 3 ignored)
• Nightly rustfmt check
• CI-equivalent Clippy check