## Summary
Individual MCP method calls may be denied (i.e. via `mcp_tool`
criterion) and Pomerium has to respond with MCP protocol error, which is
JSON-RPC error message, rather then with HTTP level error which seems to
break some MCP clients.
## Related issues
Fix
https://linear.app/pomerium/issue/ENG-2521/pomerium-does-not-return-an-mcp-error-when-a-tool-call-is-unauthorized
## User Explanation
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change doesn't create any user-facing changes, you can leave
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## Checklist
- [x] reference any related issues
- [x] updated unit tests
- [x] add appropriate label (`enhancement`, `bug`, `breaking`,
`dependencies`, `ci`)
- [x] ready for review
