fix(transport): fall back after sessionless HTTP discover rejections

[jstar0]

Summary

Some legacy streamable-HTTP servers reject server/discover in middleware with an empty or plain-text 4xx response before JSON-RPC dispatch. That prevents ClientLifecycleMode::Auto from reaching the legacy initialize path.

This change:

• Converts only sessionless server/discover client-error responses into a correlated INVALID_REQUEST response for the existing lifecycle classifier.
• Preserves 401/403 responses, 5xx responses, existing-session failures, and non-discover requests as transport errors.
• Applies the fallback to both the reqwest and Unix socket HTTP clients.
• Adds regressions for plain-text 4xx fallback and the sessionful discovery boundary.

This addresses #1040. It is complementary to #1141, which handles JSON-RPC error responses; this change covers transport-level empty, malformed, and non-JSON 4xx responses.

Testing

• cargo fmt --all -- --check
• cargo test -p rmcp --test test_streamable_http_4xx_error_body --features client,reqwest,transport-streamable-http-client-reqwest,transport-streamable-http-server
• cargo test -p rmcp --test test_discover_http_client_startup --features client,reqwest,transport-streamable-http-client-reqwest,transport-streamable-http-server
• cargo test -p rmcp --test test_unix_socket_transport --features client,server,transport-streamable-http-client-unix-socket
• cargo test -p rmcp --features client,server,reqwest,transport-io,transport-streamable-http-client-reqwest,transport-streamable-http-server (296 unit tests, all integration tests, and 39 non-ignored doctests)

Breaking Changes

None.