PipRecipeBundleResolver: pip install on the server side

[jkschneider]

What

Aligns the pip recipe install flow with the npm one. Today:

• npm: JavaScriptRewriteRpc's server-side InstallRecipes handler shells out to npm install <pkg>@<ver> --no-fund (rewrite-javascript/rewrite/src/rpc/request/install-recipes.ts:116). The JVM caller passes a package spec; the JS RPC server installs and activates.
• pip: PythonRewriteRpc's server-side handle_install_recipes doesn't install — its docstring explicitly says the package "should already be installed by the caller (e.g., via pip install --target)". The JVM-side PipRecipeBundleResolver.resolve() calls installRecipes(packageName, version) without any pip install, so when the caller hasn't pre-installed (which is the common case), _import_and_activate_package silently fails to register the recipe and a later prepareRecipe throws Recipe not found: <id>.

This PR makes the Python server install pip packages itself, matching the npm contract.

How

Python side (rewrite-python/rewrite/src/rewrite/rpc/server.py)

• New --recipe-install-dir argv parsed at startup; stored as _recipe_install_dir.
• New helper _pip_install_recipe_package(name, version, target_dir) shells out to python -m pip install --target <dir> <pkg>==<ver>, raises on non-zero exit, and primes sys.path + importlib.invalidate_caches().
• handle_install_recipes's package-spec branch invokes it when the requested package isn't already importable at the requested version, before the existing import + activate flow. Local-path branch is unchanged.

Java side (PythonRewriteRpc.java)

• One-line argv addition: --recipe-install-dir=<path> is passed to the Python subprocess when the builder's recipeInstallDir is set. The existing PYTHONPATH wiring stays so already-installed packages continue to import as before.

Test plan

• tests/rpc/test_server.py::test_handle_install_recipes_pip_installs_when_target_configured — locks the install command shape (-m pip install --target <dir> pkg==ver) and asserts the target dir lands on sys.path. Mocks subprocess.run so the test doesn't actually shell out to PyPI.
• :rewrite-python:compileJava passes.
• Existing tests/rpc/test_server.py cases still pass.

Notes for downstream callers

If you use PipRecipeBundleResolver and you weren't pre-installing recipe packages: set PythonRewriteRpc.builder().recipeInstallDir(<writable-dir>) and you're done — the server installs on demand. If you were pre-installing, no change is required: _is_package_installed short-circuits and the existing path runs.