feat: Add mirror and mirror-token inputs for custom Python distribution sources

Users who need custom CPython builds (internal mirrors, GHES-hosted forks,
special build configurations, compliance builds, air-gapped runners) could not
previously point setup-python at anything other than actions/python-versions.

Adds two new inputs:
- `mirror`: base URL hosting versions-manifest.json and the Python
  distributions it references. Defaults to the existing
  https://raw.githubusercontent.com/actions/python-versions/main.
- `mirror-token`: optional token used to authenticate requests to the mirror.

If `mirror` is a raw.githubusercontent.com/{owner}/{repo}/{branch} URL, the
manifest is fetched via the GitHub REST API (authenticated rate limit applies);
otherwise the action falls back to a direct GET of {mirror}/versions-manifest.json.

Token interaction
-----------------

`token` is never forwarded to arbitrary hosts. Auth resolution is per-URL:

  1. if mirror-token is set, use mirror-token
  2. else if token is set AND the target host is github.com,
     *.github.com, or *.githubusercontent.com, use token
  3. else send no auth

Cases:

  Default (no inputs set)
    mirror = default raw.githubusercontent.com URL, mirror-token empty,
    token = github.token.
    → manifest API call and tarball downloads use `token`.
    Identical to prior behavior.

  Custom raw.githubusercontent.com mirror (e.g. personal fork)
    mirror-token empty, token = github.token.
    → manifest API call and tarball downloads use `token`
      (target hosts are GitHub-owned).

  Custom non-GitHub mirror, no mirror-token
    mirror-token empty, token = github.token.
    → manifest fetched via direct URL (no auth attached),
      tarball downloads use no auth.
    `token` is NOT forwarded to the custom host — this is the
    leak-prevention case.

  Custom non-GitHub mirror with mirror-token
    mirror-token set, token may be set.
    → manifest fetch and tarball downloads use `mirror-token`.

  Custom GitHub mirror with both tokens set
    mirror-token wins. Used for both the manifest API call and
    tarball downloads.

docs/advanced-usage.md

@@ -525,6 +525,41 @@ Such a requirement on side-effect could be because you don't want your composite
 
 >**Note:** Python versions used in this action are generated in the [python-versions](https://github.com/actions/python-versions) repository. For macOS and Ubuntu images, python versions are built from the source code. For Windows, the python-versions repository uses installation executable. For more information please refer to the [python-versions](https://github.com/actions/python-versions) repository.
 
+#### Using a custom mirror
+
+The `mirror` input lets you point `setup-python` at a different location for CPython distributions — a personal fork of `actions/python-versions`, an internal mirror, or any server that hosts a `versions-manifest.json` at its root plus the tarballs referenced by that manifest. Default: `https://raw.githubusercontent.com/actions/python-versions/main`.
+
+The manifest is resolved as follows:
+
+- If `mirror` matches `https://raw.githubusercontent.com/{owner}/{repo}/{branch}`, the manifest is fetched via the GitHub REST API (giving you the 5000/hr authenticated rate limit when a token is present).
+- Otherwise, the action fetches `{mirror}/versions-manifest.json` via a direct HTTP GET.
+
+Authentication:
+
+- `token` is forwarded **only** to `github.com` and hosts under `*.github.com` or `*.githubusercontent.com`. It is never sent to a custom mirror.
+- `mirror-token` takes precedence over `token`: if `mirror-token` is set it is used for every authenticated request (manifest fetch and tarball downloads).
+- If `mirror-token` is empty, `token` is used when the target URL is GitHub-owned.
+- If neither applies, requests are anonymous.
+
+Point at a personal fork of `actions/python-versions` (uses the default `token`, fetched via the GitHub API):
+
+```yaml
+- uses: actions/setup-python@v6
+  with:
+    python-version: '3.12'
+    mirror: https://raw.githubusercontent.com/my-org/python-versions/main
+```
+
+Point at an internal mirror with its own credential:
+
+```yaml
+- uses: actions/setup-python@v6
+  with:
+    python-version: '3.12'
+    mirror: https://python-mirror.internal.example
+    mirror-token: ${{ secrets.PYTHON_MIRROR_TOKEN }}
+```
+
 ### PyPy
 
  `setup-python` is able to configure **PyPy** from two sources: