fix(streamable-http): map stale session 401 to status-aware error

[adam-r-kowalski]

Fixes #688.

This PR fixes streamable HTTP stale-session handling in the reqwest client: non-success HTTP responses are now surfaced as a status-aware transport error that preserves the HTTP status and response body (for example 401 Unauthorized: Session not found) instead of failing early with UnexpectedContentType(None) when Content-Type is missing.

Motivation and Context

When a stale or invalid mcp-session-id is sent to a stateful StreamableHttpService, the server correctly responds with 401 Unauthorized and a plain-text body. The client previously attempted content-type processing before handling non-success statuses, which hid the real failure as UnexpectedContentType(None).

This change makes stale-session and similar non-2xx HTTP failures actionable by preserving status/body context in UnexpectedServerResponse.

How Has This Been Tested?

Tested locally with:

cargo test -p rmcp --test test_streamable_http_stale_session --features "client server transport-streamable-http-client-reqwest transport-streamable-http-server"
cargo test -p rmcp --test test_streamable_http_priming --features "server client transport-streamable-http-server reqwest"
cargo test -p rmcp --test test_custom_headers --features "client server transport-streamable-http-client-reqwest transport-streamable-http-server"

Added regression test:

• crates/rmcp/tests/test_streamable_http_stale_session.rs
  • Verifies stale session returns HTTP 401 from server.
  • Verifies client returns StreamableHttpError::UnexpectedServerResponse containing status/body context.

Breaking Changes

None. This is a bug fix in error mapping behavior for non-success responses.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

Primary code change:

• crates/rmcp/src/transport/common/reqwest/streamable_http_client.rs
  • Early return on non-success HTTP status with UnexpectedServerResponse("HTTP {status}: {body}").

This preserves server intent for stale-session failures and avoids misleading content-type errors when error responses do not include a Content-Type header.