Fail pending Streamable HTTP requests on POST 405

[jstar0]

Summary

Fixes #144.

Fixes a Streamable HTTP edge case where a POST request with a JSON-RPC id could receive HTTP 405 immediately, but the caller would still wait until the request timeout.

Changes

• Deliver POST 405 responses to the matching pending request queue as a TransportError.
• Keep the existing lenient 405 behavior for non-waiting POSTs, SSE fallback, and session termination.
• Add a regression test that proves a pending POST 405 fails fast instead of timing out.

Verification

bundle exec rspec spec/ruby_llm/mcp/native/transports/streamable_http_spec.rb:2211
bundle exec rspec spec/ruby_llm/mcp/native/transports/streamable_http_spec.rb:247 spec/ruby_llm/mcp/native/transports/streamable_http_spec.rb:2174 spec/ruby_llm/mcp/native/transports/streamable_http_spec.rb:2397
bundle exec rspec spec/ruby_llm/mcp/native/transports/streamable_http_spec.rb
bundle exec rake rubocop