test: add unit tests for buildAllowAttribute

[suriya-93]

Adds unit tests for the buildAllowAttribute utility function, which was the only exported utility in app-bridge.ts without test coverage.

Motivation and Context

buildAllowAttribute maps McpUiResourcePermissions to an iframe allow attribute string (e.g. { clipboardWrite: {} } → "clipboard-write"). It is exported and used in the host iframe setup, but had no tests. The adjacent utilities isToolVisibilityModelOnly and isToolVisibilityAppOnly both have full coverage - this brings buildAllowAttribute in line with that pattern.

How Has This Been Tested?

• npm test — all 129 unit tests pass (8 new tests added).
• npm run prettier — all files pass formatting check.

Breaking Changes

None.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[pkg-pr-new]

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commit: 2203ea5