[Feature] Add unit tests for ScriptImporter URL construction and debugger worker URL logic

[ConnorQi01]

Summary

ScriptImporter is a core debugger component with no dedicated unit tests. It contains pure URL construction and manipulation methods that have version-dependent branching, making them good targets for focused unit tests.

Why this is useful

The prepareDebuggerWorkerURL and overridePackagerPort methods drive how the debugger connects to Metro. A bug in URL construction (wrong port, missing debugger-ui/ path for RN ≥ 0.50, wrong path for custom debuggerWorkerUrlPath) silently breaks debugging sessions. Unit tests would catch regressions here without requiring a full integration run.

Suggested scope

• Add test/debugger/scriptImporter.test.ts
• Test prepareDebuggerWorkerURL for RN < 0.50, RN ≥ 0.50, canary versions, and explicit debuggerWorkerUrlPath override
• Test overridePackagerPort with localhost URLs to verify port replacement without breaking query strings
• No need to test async I/O methods (downloadAppScript, downloadDebuggerWorker) in this issue

Evidence

• src/debugger/scriptImporter.ts (206 lines) — no corresponding test file exists
• prepareDebuggerWorkerURL has three branching paths based on semver.gte(rnVersion, "0.50.0"), isCanaryVersion, and debuggerWorkerUrlPath !== undefined
• overridePackagerPort deletes the host component specifically to force port override — a subtle invariant with no test coverage
• Aligns with the team's current pattern of adding targeted unit tests for debugger utilities (see [Feature] Add direct unit tests for DebuggerEndpointHelper #2705, [Feature] Add unit tests for NetworkInspectorServer internal logic #2694)

Validation

• npm test passes with new test cases
• Test covers all three branches of prepareDebuggerWorkerURL