fix(scripts): normalize jj and hg added paths on Windows

[claxman]

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Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: scripts/check_new_py_files.py misses added files on Windows under jj and hg #7029

Problem:
get_vcs_added_files joins jj and hg roots with os.path.join and never calls .replace(os.sep, '/'). A snippet mocked os.path to ntpath and os.sep to \\, then called get_vcs_added_files('.') with a fake jj diff --summary of A src/google/adk/agents/_jj_agent.py. Without the replace, the set is {'/workspace\\src/google/adk/agents/_jj_agent.py'}.

Solution:
Normalize the jj and hg joined paths with .replace(os.sep, '/'), matching find_py_files. git, g4, and p4 are unchanged.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

PYTHONPATH=. .venv/bin/python -m pytest -q tests/unittests/scripts/test_check_new_py_files.py

31 passed.

With scripts/check_new_py_files.py checked out from b0180620, test_get_vcs_added_files_jj[True] and test_get_vcs_added_files_hg[True] fail. They compare {'/workspace\\src/google/adk/agents/_jj_agent.py'} and {'/workspace\\src/google/adk/agents/_hg_agent.py'} to the forward-slash expected set. The posix cases ([False]) still pass.

Manual End-to-End (E2E) Tests:

Not run. No Windows host and no live jj or hg checkout.

Additional context

Claim: #7029 (comment)

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.