'Accept' Content-Types don't support wildcards

[RileyNorton]

Describe the bug
When making calls to the Streamable HTTP Transport JSON RPC endpoint, wildcards aren't supported as valid content types.

To Reproduce
Steps to reproduce the behavior:

1. Call StreamableHTTP Transport MCP Server with Accept: */* (e.g. using Postman default).
2. MCP Server responds with 401 status and the following body: Accept must contain both 'application/json' and 'text/event-stream'.

Expected behavior
I would expect the */*, text/*, and application/* values for the Accept header to imply the respective application/json and text/event-stream values. See the Mozilla docs on this header's syntax here.

Additional context
I'd love to make a short PR adding this to the switch statement at mcp/streamable.go:100. It would be my first open source contribution. 😄

[findleyr]

Thanks for the report, and the offer to send a fix! We'd gladly accept a PR.