Default to CFS for uv and azpysdk

[JennyPng]

closes #46237

• adds uv.toml which makes uv pip auto install from CFS instead of PyPI
• updates docs to provide guidance on the index feed configuration
• updates azpysdk to default to CFS unless a --pypi flag is passed
• updates azure-sdk-tools.yml to use CFS

[Copilot]

Pull request overview

This PR switches the repo/tooling default Python package index from PyPI to Azure SDK’s Central Feed Services (CFS), with opt-out behavior when PyPI is explicitly requested.

Changes:

• Add repo-root uv.toml to make uv default to the Azure SDK CFS index.
• Update azpysdk to default pip/uv installs to CFS, with a new --pypi flag to force PyPI.
• Update docs and CI (GitHub Actions) to configure pip/uv package indexes to CFS.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
uv.toml	Configures uv’s default index to the Azure SDK CFS feed.
eng/tools/azure-sdk-tools/tests/test_pypi_client.py	Adjusts PyPI-only tests to explicitly construct a client targeting pypi.org.
eng/tools/azure-sdk-tools/azpysdk/main.py	Adds CFS default index behavior and a --pypi bypass flag; restores env vars on exit.
doc/tool_usage_guide.md	Documents CFS default behavior, authentication needs, and --pypi usage.
CONTRIBUTING.md	Adds contributor guidance for CFS usage and authentication options.
.github/workflows/azure-sdk-tools.yml	Uses a new composite action to set CFS index env vars in the workflow jobs.
.github/actions/configure-package-indexes/action.yml	New composite action to export PIP_INDEX_URL and UV_DEFAULT_INDEX via GITHUB_ENV.

[Copilot]

--pypi is added as a top-level flag, but unlike --isolate/--python it can’t be specified after the subcommand (e.g., azpysdk mypy . --pypi will be rejected by argparse). If you want consistent CLI behavior with the other “global-ish” flags, consider also adding --pypi to the common parent parser (using default=argparse.SUPPRESS) so it works both before and after the subcommand.