Add support for managed installs of free-threaded Python (#8100)

Closes https://github.com/astral-sh/uv/issues/7193

```

❯ cargo run -q -- python uninstall 3.13t
Searching for Python versions matching: Python 3.13t
Uninstalled Python 3.13.0 in 231ms
 - cpython-3.13.0+freethreaded-macos-aarch64-none
❯ cargo run -q -- python install 3.13t
Searching for Python versions matching: Python 3.13t
Installed Python 3.13.0 in 3.54s
 + cpython-3.13.0+freethreaded-macos-aarch64-none
❯ cargo run -q -- python install 3.12t
Searching for Python versions matching: Python 3.12t
error: No download found for request: cpython-3.12t-macos-aarch64-none
❯ cargo run -q -- python install 3.13rc3t
Searching for Python versions matching: Python 3.13rc3t
Found existing installation for Python 3.13rc3t: cpython-3.13.0+freethreaded-macos-aarch64-none
❯ cargo run -q -- run -p 3.13t python -c "import sys; print(sys.base_prefix)"
/Users/zb/.local/share/uv/python/cpython-3.13.0+freethreaded-macos-aarch64-none
```

.github/workflows/ci.yml

@@ -655,6 +655,23 @@ jobs:
         run: |
           ./uv pip install anyio
 
+      - name: "Install free-threaded Python via uv"
+        run: |
+          ./uv python install 3.13t
+          ./uv venv -p 3.13t --python-preference only-managed
+
+      - name: "Check version"
+        run: |
+          .venv/bin/python --version
+
+      - name: "Check is free-threaded"
+        run: |
+          .venv/bin/python -c "import sys; exit(1) if sys._is_gil_enabled() else exit(0)"
+
+      - name: "Check install"
+        run: |
+          ./uv pip install anyio
+
   integration-test-pypy-linux:
     timeout-minutes: 10
     needs: build-binary-linux