Declare pip runtime dependency

[JamBalaya56562]

Summary

• declare pip as a PlatformIO Python dependency so pipx/uvx/uv tool environments install it explicitly
• remove the duplicate hard-coded pip upgrade target from upgrade_pip_dependencies()
• add focused tests for the dependency list and upgrade command construction

Root cause

PlatformIO invokes python -m pip at runtime for tool/Python package installation, but pip was not listed in get_pip_dependencies(). Isolated runners such as pipx, uvx, and uv tool install only install declared dependencies, so those environments can omit the pip module and fail with No module named pip.

Fixes #5418.
Related to #5305.

Validation

• C:\Users\Jam\AppData\Local\mise\installs\uv\0.11.26\uv.exe run --with pytest --with-editable . -- python -m pytest tests/misc/test_dependencies.py
• C:\Users\Jam\AppData\Local\mise\installs\uv\0.11.26\uv.exe run --with-editable . -- python -c "import importlib.metadata as md; reqs=md.requires('platformio') or []; print([r for r in reqs if r == 'pip']); print(len(reqs))"
• mise exec uv@0.11.26 -- uv run --with pytest --with-editable . -- python -m pytest tests/misc/test_dependencies.py

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