Simplify packed KV cache grouping

[LucasWilkinson]

Suggestion PR into release/qwen38next (vllm-project#53896) — not a standalone contribution to vLLM, so no duplicate-PR concern; it proposes cleanups to code introduced by that PR.

Changes

• _get_packed_kv_cache_groups: add a docstring, merge the duplicated group-emission branches into a single loop with a num_groups == 1 shortcut, collapse the mixed-bucket repeat list into a min_repeats_per_group scalar with an explanatory comment, and drop the defensive getattr on cache_config.
• Rename the "layer tuple" terminology: UniformTypeKVCacheSpecs.get_num_layer_tuples → get_max_layers_per_page_size (mechanically accurate name), plus clearer local names (pattern_repeats, repeats_per_group, page_size_layers). Document the 1:1 one-layer-per-page-size pattern assumption on the balanced check.
• Gate the DeepseekV4 eagle-group annotation at the call site via _is_deepseek_v4_eagle (checks hf_config.model_type) instead of sniffing model_version off the merged MLA spec inside the helper.
• Test updates: packing tests now assert manager-level grouping semantics rather than exact group ordering; test_qwen4_exp.py follows the compress_ratio → tokens_per_state / num_states spec field names.

Note one behavior nuance: with the num_groups == 1 shortcut, a balanced mixed bucket keeps its original spec-dict layer order instead of the zip-interleaved order. These coincide when layers are registered in model layer order (the normal case, covered by the placement tests), flagging in case an edge case relies on the reorder.

Testing

pytest tests/v1/core/test_contiguous_kv_packing.py tests/v1/core/test_kv_cache_utils.py tests/v1/spec_decode/test_qwen4_exp.py
106 passed

AI assistance (Claude Code) was used for this change; every line was human-reviewed.

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