fix: improve SSE event handling to gracefully ignore unrecognized events

[tzolov]

This change improves client resilience by gracefully handling unknown events instead of failing, allowing clients to continue processing valid events even when encountering unexpected ones.

• Replace McpError exceptions with debug/warning logs for unrecognized SSE event types
• Continue processing instead of failing when encountering unknown SSE events
• Update transport implementations:
  • WebClientStreamableHttpTransport: return empty tuple instead of throwing
  • WebFluxSseClientTransport: complete stream instead of erroring
  • HttpClientSseClientTransport: call sink.success() instead of sink.error()
  • HttpClientStreamableHttpTransport: return empty Flux for unknown events

This improves client resilience when servers send non-standard or future SSE event types.

Resolves #272 , #223 , #93, #421

Improve SSE event handling to gracefully ignore unrecognized events instead of throwing errors

Motivation and Context

Previously, when MCP clients received unrecognized Server-Sent Events (SSE) from servers, they would throw McpError exceptions and terminate the connection. This created fragility when:

• Servers send non-standard SSE event types
• Future MCP versions introduce new SSE event types that older clients don't recognize
• SSE comment lines (starting with :) are sent, which should be ignored per the SSE specification

How Has This Been Tested?

• Added unit test testCommentSseMessage() to verify SSE comment lines are properly ignored
• Existing transport tests continue to pass, ensuring backward compatibility
• Tested scenarios include:
  • SSE comment lines (: prefixed lines)
  • Unknown SSE event types
  • Mixed streams with both recognized and unrecognized events

Breaking Changes

No breaking changes. This is a backward-compatible improvement that makes clients more resilient. Existing functionality remains unchanged.

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

Implementation Details:

• WebClientStreamableHttpTransport: Returns empty tuple instead of throwing
• WebFluxSseClientTransport: Completes stream with warning log instead of erroring
• HttpClientSseClientTransport: Calls sink.success() with debug log instead of sink.error()
• HttpClientStreamableHttpTransport: Returns empty Flux for unknown events

[sunyuhan1998]

That's great! I think this PR can solve many issues.

[tzolov]

Rebased, squashed and merged at c3a0b18