Serve fully cached repos without a Hub round-trip

[Vlor999]

Proposed changes

Fixes #1649.

_download() called snapshot_download() without local_files_only, so a repo
that is already fully cached still paid a Hub API round-trip to check for
updates before serving from the cache. load() calls _download() twice — once
for weights, once for tokenizer files — so every invocation paid it twice, for
the CLI and for library users alike, not only on first download.

This tries local_files_only=True first and falls back to the existing online
call on LocalEntryNotFoundError, so behaviour is unchanged whenever the cache
cannot satisfy the request.

Why

huggingface_hub 1.26.0, warm cache, median of 5 calls on an already-cached repo:

	per call	per load() (2 calls)
before (network check)	147.2 ms	~294 ms
after (local first)	0.26 ms	~0.5 ms
	565x

Reproducer is in #1649. The absolute number is latency to the Hub, so it grows
on a slow link — and on a flaky one the current code blocks until the check
times out, where this returns immediately.

How

• Try the cache first, fall back on LocalEntryNotFoundError only.
• Gated on _supports_local_files_only, because ModelScope's snapshot_download
  takes no such argument. On ModelScope the behaviour is byte-for-byte unchanged.
• hf_repo_to_path() in the same file already uses local_files_only=True, so
  this follows an established pattern rather than introducing one.

Alternative rejected: catching a broader exception. LocalEntryNotFoundError
is what a cache miss raises (verified against 1.26.0); catching more would
silently swallow real Hub errors and turn them into a redundant second attempt.

Validation

python -m unittest tests.test_utils      # 11 passed
pre-commit run --files mlx_lm/utils.py tests/test_utils.py   # black, isort

Two tests added:

• test_cached_repo_skips_network — a cached repo results in exactly one
  snapshot_download call, made with local_files_only=True.
• test_falls_back_to_network_when_not_cached — a LocalEntryNotFoundError
  produces a second call without local_files_only.

I checked the tests are not vacuous by flipping the new argument to False:
both fail. With the fix in place, all 11 pass.

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Supersedes #1571, which was opened against an older main. This is the same fix
rebuilt on current main, with the measurement and tests added.