docs(cli): sync the serve --tp-size family list with generate and mlxcel-server

## Summary

The `--tp-size` help text on `mlxcel serve` lists a stale tensor-parallel family set. It omits Qwen3.5 text and Gemma 4 text, which the other two spellings of the same flag already list.

## Background

The tensor-parallel family list appears in three places, and `docs/distributed.md` names the `generate` and `mlxcel-server` sites as the source of truth. The `ServeArgs` doc comment is the odd one out, so `mlxcel serve --help` advertises fewer supported families than the runtime actually handles.

## Proposed Solution

Sync the one doc-comment line in `ServeArgs` with the other two sites so all three `--tp-size` help texts list the identical family set.

## Implementation Notes

- `src/main.rs:2135-2136` (`ServeArgs`) reads "dense Llama, Qwen2/2.5, Qwen3, Gemma 3 text, ERNIE 4.5, and Hunyuan v1 Dense".
- `src/main.rs:854-856` (`TensorParallelOptions`, used by `mlxcel generate`) and `src/bin/mlx_server.rs:1422-1425` (`mlxcel-server`) both read "dense Llama, Qwen2/2.5, Qwen3, Qwen3.5 text, Gemma 3 text, Gemma 4 text, ERNIE 4.5, and Hunyuan v1 Dense".
- `docs/distributed.md:43-47` names those two files as the source of truth and quotes the longer list.

## Acceptance Criteria

- [ ] All three `--tp-size` help texts list the identical family set

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## Original Suggestion

### Title: docs(cli): sync the serve --tp-size family list with generate and mlxcel-server

The `--tp-size` help text on `mlxcel serve` lists a stale tensor-parallel family set — it is missing Qwen3.5 text and Gemma 4 text, which the other two spellings of the same flag already list.

## Evidence

- `src/main.rs:2134-2136` (`ServeArgs`) — "dense Llama, Qwen2/2.5, Qwen3, Gemma 3 text, ERNIE 4.5, and Hunyuan v1 Dense"
- `src/main.rs:853-856` (`TensorParallelOptions`, used by `mlxcel generate`) and `src/bin/mlx_server.rs:1422-1425` (`mlxcel-server`) — both say "dense Llama, Qwen2/2.5, Qwen3, Qwen3.5 text, Gemma 3 text, Gemma 4 text, ERNIE 4.5, and Hunyuan v1 Dense"
- `docs/distributed.md:43-47` names those two files as the source of truth and quotes the longer list — `mlxcel serve --help` is the odd one out

## Suggested fix

Sync the one doc-comment line in `ServeArgs` with the other two sites.

## Acceptance criteria

- [ ] All three `--tp-size` help texts list the identical family set

Compare the ServeArgs doc comment at src/main.rs:2135-2136 with the longer --tp-size text in src/main.rs:854-856 and src/bin/mlx_server.rs:1422-1425; docs/distributed.md identifies the latter sites as the source of truth. Sync the serve help text so all three --tp-size help texts list the identical family set, then verify the acceptance criterion.

Written by the indexing model from the issue text.