feat: Add mirror and mirror-token inputs for custom Python distribution sources (#1302)

* 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.

* fix: address mirror review feedback

- scope mirror-token to the mirror host and send it verbatim
- route non-repo mirrors straight to the URL fetch instead of throwing
- authenticate the manifest fetch
- warn on slash branches, and on mirror with PyPy/GraalPy
- memoize mirror validation
- exercise the direct-URL path in the E2E job

Addresses https://github.com/actions/setup-python/pull/1302#issuecomment-5202618946

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* fix: correct mirror warnings, auth scoping, and integration coverage

- only warn about PyPy/GraalPy mirror when a custom mirror is set; the
  action.yml default made the warning fire on every run
- accept the refs/heads/{branch} raw URL form so it routes via the REST
  API instead of tripping the slash-branch warning
- scope mirror-token to the full mirror origin (scheme+host+port) so it
  can't leak to a same-host http download_url
- make an invalid mirror fatal on the auth path, matching getManifestUrl
- fix warning/docs that wrongly claimed the raw fallback is anonymous
- force a manifest fetch in the mirror integration job (check-latest) so
  it actually contacts the mirror instead of using the preinstalled cache

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

src/setup-python.ts

@@ -2,6 +2,7 @@ import * as core from '@actions/core';
 import * as finder from './find-python.js';
 import * as finderPyPy from './find-pypy.js';
 import * as finderGraalPy from './find-graalpy.js';
+import {isMirrorCustomized} from './install-python.js';
 import * as path from 'path';
 import * as os from 'os';
 import {fileURLToPath} from 'url';
@@ -23,6 +24,21 @@ function isGraalPyVersion(versionSpec: string) {
   return versionSpec.startsWith('graalpy');
 }
 
+// `mirror` only redirects CPython distributions. PyPy and GraalPy resolve from
+// downloads.python.org and the GitHub releases API respectively, so warn rather
+// than let the input look like it applied. Only warns when the user actually
+// set a custom mirror: action.yml gives `mirror` a default, so a plain
+// getInput() check would fire on every pypy-*/graalpy-* run.
+function warnIfMirrorUnsupported(versionSpec: string) {
+  if (!isMirrorCustomized()) {
+    return;
+  }
+  const implementation = isPyPyVersion(versionSpec) ? 'PyPy' : 'GraalPy';
+  core.warning(
+    `The 'mirror' input only applies to CPython distributions and is ignored for ${implementation} ('${versionSpec}'), which is downloaded from its own upstream source.`
+  );
+}
+
 async function cacheDependencies(cache: string, pythonVersion: string) {
   const cacheDependencyPath =
     core.getInput('cache-dependency-path') || undefined;
@@ -102,6 +118,7 @@ async function run() {
       core.startGroup('Installed versions');
       for (const version of versions) {
         if (isPyPyVersion(version)) {
+          warnIfMirrorUnsupported(version);
           const installed = await finderPyPy.findPyPyVersion(
             version,
             arch,
@@ -114,6 +131,7 @@ async function run() {
             `Successfully set up PyPy ${installed.resolvedPyPyVersion} with Python (${installed.resolvedPythonVersion})`
           );
         } else if (isGraalPyVersion(version)) {
+          warnIfMirrorUnsupported(version);
           const installed = await finderGraalPy.findGraalPyVersion(
             version,
             arch,