Fix pip --python flag ordering in PYAPP-mode plugin dependency sync

[aryansk]

Summary

When a plugin declares dependencies and hatch needs to sync them, the built pip/uv command appended install --disable-pip-version-check --python <executable> after the subcommand name — the correct order for uv, but not for pip.

pip requires global options like --python to come before the subcommand:

# uv (works):
uv pip install --disable-pip-version-check --python <exe> foo

# pip (silently ignores --python, installs into wrong env):
pip install --disable-pip-version-check --python <exe> foo
# correct:
pip --python <exe> install --disable-pip-version-check foo

This is only reachable in PYAPP mode (PYAPP + PYAPP_COMMAND_NAME set), where the pip backend is used — uv's ordering was already correct. The flag was being parsed as a pip install option, which pip rejects unknown --python for, so the dependency sync failed (or worse, installed into the wrong environment when a compatible Python was found).

Changes

• Split the argument construction per backend:
  • pip (PYAPP mode): --python <exe> before install
  • uv: unchanged (install before --python)
• Added regression tests for both branches asserting the exact command ordering.

Test Plan

• pytest tests/cli/test_application.py — 2 passed
• Full suite: 2511 passed; the 25 failures (flit-core / build-frontend / venv) reproduce identically on clean master and are environment-related.
• ruff check and ruff format --check clean on changed files.

Fixes #2389