Improve scripts/pr_file_map.py observability and output metadata - #15317

Describe your change

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• Documentation change?

pr_file_map.py now emits structured run-status counts to stderr and includes generation metadata plus summary counts in markdown output. It also migrates path existence checks from os.path to pathlib.Path and documents the updated behavior in the module docstring.

• Path handling

  • Replaced os.path.exists(path) with Path(path).exists().

• Runtime status (stderr)

  • Added counts for:
    • PRs returned by get_open_prs()
    • files returned by get_pr_files()
    • existing vs missing files

• Report output (stdout markdown)

  • Added top-level metadata:
    • script filepath (Path(__file__).resolve())
    • UTC generation timestamp
    • PR/file/existing/missing counts
  • Preserved explicit no-open-PR messaging while still outputting summary metadata.

print(f"- Script: `{Path(__file__).resolve()}`")
print(f"- Generated (UTC): `{datetime.now(UTC).isoformat()}`")
print(f"- Number of PRs: `{pr_count}`")
print(f"- Number of files: `{file_count}`")
print(f"- Existing files: `{existing_count}`")
print(f"- Missing files: `{missing_count}`")

Checklist

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation.