[Core][KV Connector] Avoid hybrid KV load failure crash

[chfeng-cs]

Purpose

Fix #45474.
Fix a scheduler crash in KV load failure recovery for hybrid KV cache models. The existing path assumed get_block_ids() returned a single KV cache group, but hybrid models return one block-id list per group.

This PR keeps the existing precise recovery behavior for single-group requests and adds a conservative fallback for multi-group requests: if any invalid block belongs to the request, reset its computed KV prefix and reschedule it for recomputation.

This is a tradeoff for the failure path. Precise recovery for hybrid KV would need group-aware invalid-block metadata and a way to map failed blocks back to the earliest safe token prefix across all KV cache groups, which is a larger change. This PR instead prioritizes correctness and keeping the engine alive over minimizing recomputation. Since KV load failures are expected to be uncommon, the extra recomputation only applies on the error path.

Happy to work on precise recovery for hybrid KV if maintainers think it is necessary.

Test Plan

Run the focused invalid-block recovery tests and ruff on the modified files.

Test Result

pytest tests/v1/kv_connector/unit/test_invalid_blocks_correctness.py::test_hybrid_invalid_blocks_fallback_recomputes_from_beginning -q
pytest tests/v1/kv_connector/unit/test_invalid_blocks_correctness.py::test_sync_recompute_blocks_not_freed_for_running_requests tests/v1/kv_connector/unit/test_invalid_blocks_correctness.py::test_sync_fail_invalid_blocks_evicted tests/v1/kv_connector/unit/test_invalid_blocks_correctness.py::test_hybrid_invalid_blocks_fallback_recomputes_from_beginning -q
ruff check vllm/v1/core/sched/scheduler.py tests/v1/kv_connector/unit/test_invalid_blocks_correctness.py

All passed.

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Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.