fix: allow creating new files in nested non-existent directories

[Juwan-Hwang]

Problem

When validating paths for new files that have not yet been created, validatePathWithinRepo fell back to calling realpath(parentDir) on the immediate parent directory.

If a new file is being created inside nested directories that do not yet exist (for example src/components/button/index.tsx where neither components/ nor button/ has been created yet), realpath(parentDir) threw ENOENT, which was caught and caused the function to incorrectly reject the valid path with Path '...' resolves outside the repository root.

Solution

Update validatePathWithinRepo to walk up the directory tree to find the closest existing ancestor directory:

• Recursively inspect parent directories until an existing ancestor directory is found.
• Verify that the resolved ancestor directory resides within resolvedRoot.
• Return initialPath for the non-existent target path once the ancestor is validated.

Testing

• Added unit tests for new files in nested non-existent directories under existing directories (src/components/button/index.tsx).
• Added unit tests for new files in deeply nested non-existent directories from repo root (deeply/nested/non/existent/dir/file.txt).
• Verified path traversal attacks (../outside/...) and escaping symlinks continue to be correctly rejected for non-existent nested paths.
• Verified valid internal symlinks to directories continue to accept nested new files.
• All path validation unit tests pass (27 pass, 0 fail).
• All format and TypeScript checks pass (bun run format:check, bun run typecheck).

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