fix: #699 Forward fetch parameter to SSEClientTransport in MCPServerSSE

[aasullivan]

Summary

Fixes #699

The MCPServerSSE class was accepting a fetch parameter but not forwarding it to
the underlying SSEClientTransport, preventing custom fetch implementations from
being used with SSE-based MCP servers.

Changes

• Added fetch?: any parameter to MCPServerSSEOptions interface
• Updated NodeMCPServerSSE.connect() to forward the fetch parameter to
  SSEClientTransport
• Added tests to verify custom fetch is properly forwarded
• Added test to verify backward compatibility (SSE server works without custom fetch)

Impact

This bug made it impossible to:

• Add authentication headers to SSE MCP server requests
• Customize request behavior (timeouts, retries, logging)
• Use corporate proxies or custom network configurations

Now MCPServerSSE works consistently with MCPServerStreamableHttp, which already
correctly forwards the fetch parameter.

Testing

• All existing tests pass (645 tests)
• Added new test suite for NodeMCPServerSSE with 2 tests
• Verified linting passes
• Verified build completes successfully

Checklist

• Tests added/updated
• Build passes (pnpm build)
• Tests pass (pnpm test)
• Linting passes (pnpm lint)
• Changeset created (pnpm changeset)

This format:

• ✅ Links the related issue (MCPServerSSE doesn't forward fetch parameter to SSEClientTransport #699)
• ✅ Summarizes the changes clearly
• ✅ Explains the impact/why it matters
• ✅ Shows what testing was done
• ✅ Includes a checklist showing you followed all guidelines

[changeset-bot]

🦋 Changeset detected

Latest commit: 5de21b6

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 5 packages

Name	Type
@openai/agents-core	Patch
@openai/agents-extensions	Patch
@openai/agents-openai	Patch
@openai/agents-realtime	Patch
@openai/agents	Patch

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