support 405 response from servers that don’t support streaming

[stallent]

Motivation and Context

Some MCP servers do not support streaming from the GET. They should return a 405 to denote that lack of support. Currently the client retries on errors and should not retry if it knows the server does not support it.

How Has This Been Tested?

Stood up a server using the latest TS SDK from Anthropic configured to not support

Breaking Changes

none

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

[mattt]

Thank you, @stallent!