fix: improve streamable HTTP session reinitialization

[tzolov]

Implements the MCP spec guidelines for streamable HTTP (re)initialization:

• Server MAY terminate session and MUST respond with HTTP 404 for terminated session IDs
• Client MUST start new session when receiving HTTP 404 for requests with session ID

Changes:

• Replace generic McpError with McpTransportException for transport-layer errors
• Only throw McpTransportSessionNotFoundException when session ID is present in request (per spec: 404 with session ID means session terminated, without means general error)
• Enhance error messages with more context (status codes, response events)
• Use RuntimeException for non-transport specific SSE endpoint failures
• Ensure consistent error handling across HTTP client transports

Fixes #459

Motivation and Context

This change addresses issue #459 and implements the MCP specification requirements for streamable HTTP (re)initialization:

• Spec Compliance: The MCP spec states that servers MAY terminate sessions and MUST respond with HTTP 404 for terminated session IDs. Clients MUST then start new sessions when receiving HTTP 404 for requests containing session IDs.
• Incorrect Error Handling: Previously, the code would throw McpTransportSessionNotFoundException for any 404/400 response, even when no session ID was present, which didn't align with the spec.
• Generic Exceptions: The use of generic McpError for transport-layer issues made it difficult to distinguish between different error types and handle them appropriately.

This fix ensures proper session reinitialization flow where clients can detect terminated sessions (404 with session ID) and start fresh sessions without an ID.

How Has This Been Tested?

• Tested session termination scenarios where server returns 404 for valid session IDs
• Verified that 404 responses without session IDs are handled as general transport errors
• Tested 400 BAD REQUEST responses with and without session IDs
• Verified error message clarity and exception type consistency across different transport implementations
• Existing test suite passes with the new error handling logic

Breaking Changes

Minor breaking changes for users who catch specific exception types:

• Code catching McpError for transport errors will need to catch McpTransportException instead
• Error handling logic that assumes all 404/400 responses are session errors will need updating

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

[tzolov]

Rebase, squashed and merged at 6c38f37