fix: handle array paths from VSCode terminal profiles

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Summary

This PR fixes the issue where VSCode's terminal profile configuration may return path as an array instead of a string, causing the error "The 'path' argument must be of type string. Received an instance of Array".

Problem

As reported in #7695, the VSCode API may return profile.path as an array (e.g., ["C:\\Program Files\\PowerShell\\7\\pwsh.exe", "pwsh.exe"]) instead of a string. This causes the isShellAllowed function to fail since it expects a string parameter.

Solution

1. Updated type definitions to reflect that path can be string | string[]
2. Added normalizeShellPath helper to safely extract the first element from array paths
3. Created validateShellPath export as a robust public API for shell validation that handles:
   • String paths (normal case)
   • Array paths from VSCode API (uses first element)
   • Null/undefined values
   • Empty arrays
   • Nested arrays (recursive handling)
   • Invalid types (safely rejects)
4. Refactored internal validation into isShellAllowedInternal for cleaner separation
5. Maintained backward compatibility with deprecated isShellAllowed function

Testing

• Added comprehensive test suite with 22 test cases covering all edge cases
• All existing tests pass without regression
• Tests verify proper handling of arrays, strings, null values, and platform-specific behavior

Changes

• src/utils/shell.ts: Enhanced shell validation with array support
• src/utils/__tests__/shell.test.ts: Comprehensive test coverage

Fixes #7695

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Important

Fixes handling of array paths in VSCode terminal profiles by normalizing paths and updating shell retrieval logic.

• Behavior:
  • Fixes handling of path as string[] in VSCode terminal profiles, using first element if array.
  • Updates getShell() in shell.ts to handle array paths and fall back to defaults if necessary.
• Functions:
  • Adds normalizeShellPath() to extract first element from array paths.
  • Refactors getWindowsShellFromVSCode(), getMacShellFromVSCode(), and getLinuxShellFromVSCode() to use normalizeShellPath().
• Testing:
  • Adds tests in shell.spec.ts for array paths, empty arrays, and fallback scenarios.
  • Ensures getShell() handles all edge cases, including invalid and nested arrays.

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[roomote]

Reviewing my own code again. It's like grading my own homework, except the teacher is also me.