Expose a Python "identity" output (#1036)

https://github.com/pyca/cryptography/pull/15572#discussion_r3913508686
has the context for this: TL;DR our current `python-version` output
mirrors the "request" version exactly, which means that it's
insufficient for any downstream that needs to manage its own cache keys
(since caches shouldn't be shared across release candidates, but the `uv
python` request version doesn't include RC numbers).

The first commit here was my attempt to fix this by exposing the runtime
Python version, but this too is imprecise: the runtime version doesn't
indicate the interpreter variant (e.g. freethreading), which is also
important to capture in the cache identity.

My solution here is to expose `python-runtime-id`, which is just the
`key` of the active Python version from `uv python list
--output-format=json`.

---------

Signed-off-by: William Woodruff <william@yossarian.net>

.github/workflows/test.yml

@@ -347,9 +347,14 @@ jobs:
           if [ "$PYTHON_VERSION" != "3.13.1t" ]; then
             exit 1
           fi
+          if [ -n "$PYTHON_RUNTIME_ID" ]; then
+            echo "python-runtime-id should be empty without environment activation"
+            exit 1
+          fi
         shell: bash
         env:
           PYTHON_VERSION: ${{ steps.setup-uv.outputs.python-version }}
+          PYTHON_RUNTIME_ID: ${{ steps.setup-uv.outputs.python-runtime-id }}
       - run: uv sync
         working-directory: __tests__/fixtures/uv-project
 
@@ -439,6 +444,15 @@ jobs:
               raise SystemExit(f"Python is not running from custom venv: {sys.executable}")
           PY
         shell: bash
+      - name: Verify Python runtime ID from custom venv
+        run: |
+          case "$PYTHON_RUNTIME_ID" in
+            cpython-3.13.1+freethreaded-*) ;;
+            *) echo "Wrong Python runtime ID: $PYTHON_RUNTIME_ID"; exit 1 ;;
+          esac
+        shell: bash
+        env:
+          PYTHON_RUNTIME_ID: ${{ steps.setup-uv.outputs.python-runtime-id }}
 
   test-activate-environment-no-project:
     runs-on: ubuntu-latest