design: add MCP inspector authentication design doc

[R-Lawton]

Summary

• adds design doc for MCP inspector authentication covering three auth modes: no auth, custom header, and OIDC
• OIDC approach uses the MCP SDK's OAuth flow with dynamic client registration (RFC 7591) and full-page redirect, adapted from the upstream MCP Inspector
• follows up on the "Future Considerations" section from the MCP client proxy design

Test plan

• review design doc for completeness and accuracy

🤖 Generated with Claude Code

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch design/mcp-inspector-auth

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