Deepak/fix issue 1031/prevent python symlink escape

[deepakdp17]

Summary

• Resolves symlinks using filepath.EvalSymlinks before reading or writing requirements files, so a symlinked requirements.txt pointing outside the repository is rejected instead of silently followed
• Adds a shared ValidateFileWithinDir utility that other package handlers can reuse
• Hardens descriptor file discovery (GetAllDescriptorFilesFullPaths) to skip symlinks that resolve outside the working directory, covering NuGet, Pnpm, Conan, and others

Why

Frogbot's Python handler validated file paths using string prefix checks, but never resolved symlinks. In CI environments, a malicious symlink like requirements.txt -> /etc/shadow would pass the path check and still be read or overwritten. This is the issue described in #1031.

What changed

utils/pathutils.go — New ValidateFileWithinDir function that resolves symlinks on both the file and the allowed directory, then checks containment
utils/pathutils_test.go — 7 tests covering regular files, subdirectories, symlink escape, symlinks within the directory, path traversal, nonexistent files, and directory symlink escape
packagehandlers/pythonpackagehandler.go — tryReadRequirementFile and handlePip now validate paths through the new utility before any read or write
packagehandlers/commonpackagehandler.go — GetAllDescriptorFilesFullPaths validates each discovered file and logs a warning if it skips one

• All tests passed. If this feature is not already covered by the tests, I added new tests.
• Project compiles cleanly
• No lint errors introduced
• Existing TestUpdateDependency pip/poetry/pipenv cases should still pass (requires JFrog server credentials)
• Manual test: create a symlinked requirements.txt pointing outside the repo and confirm Frogbot rejects it
• This pull request is on the dev branch.
• I used gofmt for formatting the code before submitting the pull request.

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Fixes #1031