fix(streamable-http-client): handle error responses to initialize

[gocamille]

Fixes #1360

expect_initialized now stops draining an SSE response on an error as well as a result, so the error reaches the service layer and serve() returns ClientInitializeError::JsonRpcError, the same as it already does for application/json.

perform_reinitialization now returns an error when the re-sent initialize gets an error reply, before it sends notifications/initialized. The worker's existing recovery-failure path then fails the pending requests, so nothing is retried without a session. I reused UnexpectedServerResponse for this rather than adding a variant, since recovery errors reach callers as TransportSend anyway and the message carries the server's code and text.

The new tests in test_streamable_http_initialize_error.rs use an axum mock server:

• initialize error over SSE, then the stream ends
• initialize error over SSE, stream held open
• re-initialize answered with a JSON error
• re-initialize answered with an SSE error (stream held open)

The two re-init tests check the full list of POSTs the server receives, so they fail if notifications/initialized or the retried request goes out after the error.

Validation

• All four new tests fail on main and pass with the change.
• cargo test -p rmcp --all-features: passed.
• cargo clippy --all-targets --all-features -- -D warnings and cargo +nightly fmt --all -- --check: clean.

[github-actions]

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[DaleSeo]

Thanks for the fix, @gocamille! I only have a minor suggession.