fix(kv cache): preserve MLA quantization metadata when unifying page sizes

[voipmonitor]

Problem

unify_kv_cache_spec_page_size() reconstructed MLAAttentionSpec objects without carrying kv_quant_mode. A quantized MLA cache could silently lose its quantization metadata while page sizes were normalized, causing downstream allocation or backend selection to treat it as the default mode.

Fix

Propagate kv_quant_mode into the reconstructed spec and add coverage for heterogeneous MLA/SWA/indexer cache groups.

This is the current-FF replacement for the metadata fix previously embedded in #88.

Validation

• 2 targeted KV-cache utility tests passed
• ruff check and ruff format --check on changed files
• git diff --check

[coderabbitai]

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Review details

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: ec7072af-f7ce-4342-b4b0-12e0e2a7a443

📥 Commits

Reviewing files that changed from the base of the PR and between e4879e1 and f281b35.

📒 Files selected for processing (2)

• tests/v1/core/test_kv_cache_utils.py
• vllm/v1/core/kv_cache_utils.py

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/ff-kv-quant-mode-propagation-20260717

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