Add SDK MCP OAuth host token handlers

[roji]

Summary

• Add SDK-facing MCP OAuth host-token callbacks across Node, Python, Go, .NET, Java, and Rust.
• Add generated RPC/event surfaces for session.mcp.oauth.handlePendingRequest, mcp.oauth_required, and mcp.oauth_completed.
• Register mcp.oauth_required interest only when the high-level MCP auth callback is configured.
• Add Node E2E coverage with a fake OAuth-protected HTTP MCP server fixture.

Notes

• Draft while the corresponding runtime work lands: github/copilot-agent-runtime#10195.
• The new Node MCP OAuth E2E uses the normal SDK construction-time path and requires the updated runtime behavior/package.

Validation

• Node targeted MCP auth unit tests passed.
• Node typecheck passed.
• Prettier check passed.
• Node MCP OAuth E2E passes against the sibling runtime branch; with the current packaged runtime it does not connect the OAuth test server yet.
• Earlier validation in this session covered .NET build, Rust targeted tests, Python compile, and Node build/unit tests; Go/Java full validation were blocked locally by missing toolchains.