macOS Apple Silicon: C++ Segfault during test shutdown due to TF GCS client (tokenizer caching)

**System Information:**
- OS: macOS (Apple Silicon)
- Python Version: 3.12
- Environment: Local `pytest` execution

**Bug Description:**
When running the `gemma/gm/text` test suite locally on macOS Apple Silicon, the `pytest-xdist` workers crash with the following unhandled C++ exception during interpreter shutdown:
`libc++abi: terminating due to uncaught exception of type std::__1::system_error: mutex lock failed: Invalid argument`

**Root Cause:**
The issue stems from `gemma/gm/utils/_file_cache.py`. Currently, `maybe_get_from_cache` does not actually download and save the tokenizer model to the local cache directory if it is missing. Instead, it returns the `gs://` path, causing `epath.Path('gs://...').read_bytes()` to be executed.

This directly invokes the TensorFlow C++ GCS client via gRPC to read the bytes over the network. On macOS Apple Silicon, there is a known bug where tearing down the `SentencePieceProcessor` and the TensorFlow C++ GCS client threads during interpreter shutdown causes a mutex lock failure and segfaults the process.

**Proposed Fix:**
We should update `_file_cache.py` to intercept `gs://` paths, download them over standard HTTP (e.g., using `urllib.request`), and save them to the local `~/.gemma/tokenizer/` cache directory *before* returning the path.

This ensures `epath` only ever performs local disk I/O, completely avoiding the C++ GCS client and preventing the crash on macOS. As an added benefit, this introduces true local caching, meaning the test suite (and user code) won't have to download the tokenizer over the network on every single run.

I will be raising a PR shortly with this fix!