fix: silently hide MCP UI on errors

[RomneyDa]

Summary

• When an MCP UI app encounters errors (e.g. it can't handle a particular tool's data), the UI now silently hides instead of showing broken/confusing content like "Unable to parse tool results"
• Wraps sendToolInput and sendToolResult bridge calls in try/catch so failures hide the UI gracefully
• Listens for error/critical log messages from the MCP app via the bridge and hides UI in response

Context

MCP servers may set _meta.ui.resourceUri on tools where the UI app can't actually handle that tool's data. The detection of whether a tool has UI is correct (we trust the server's metadata), but we need to be resilient when the UI app can't render.

Test plan

• Verify MCP tools with working UI still render correctly
• Verify MCP tools where the UI app can't handle the data no longer show error messages — the UI simply doesn't appear
• Verify bridge connection failures result in no UI rather than a red error box

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Summary by cubic

Silently hide the MCP UI when the embedded app errors, instead of showing confusing error boxes. This makes incompatible tool UIs fail closed while keeping functional UIs unaffected.

• Bug Fixes
  • Return null on MCP app errors or error/critical logs from the bridge.
  • Wrap sendToolInput and sendToolResult in try/catch; on failure, set error and hide UI.
  • Avoid rendering the red error box; no UI appears when the app can’t render.

^{Written for commit fdede37. Summary will update on new commits.}

[cubic-dev-ai]

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