feat: add conformance tests for SEP-990

[sagar-okta]

Adds conformance tests for SEP-990 which introduces SSE polling via server-side disconnect.

Motivation and Context

How Has This Been Tested?

Tested with modelcontextprotocol/typescript-sdk#1328

Breaking Changes

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

[pcarleton]

🙌 thanks for this! will aim to get you comments by tomorrow, but i'm very excited to have a test for this.

one thing missing from this tool currently is marking tests as optional. Current state is MUST = FAIL, SHOULD = WARNING, and for tiering we want SHOULD to be required (want the best SDK's to do the SHOULD's).

extensions are a new axis, where we want to say "this feature is optional, but within the test, the same MUST/SHOULD logic applies" which will apply here and to client credentials which is currently not differentiated.

I'm just noting the concept we need to add, not in the scope of this PR.