[BugFix] Fix AscendStore lookup TypeError at ZMQ boundary

[jichaofdu]

What this PR does / why we need it?

LookupKeyClient serializes vLLM BlockHash values as hexadecimal strings for
the ZMQ lookup RPC. After msgpack decoding, LookupKeyServer forwarded those
strings directly to KVPoolWorker.lookup_scheduler(), although downstream
grouped-hash code expects bytes-like BlockHash elements.

In a v0.23.0rc1-based AscendStore deployment with the grouped-hash changes from
#12814 and #13110 backported, this produced:

TypeError: sequence item 0: expected a bytes-like object, str found

The lookup exception handler then returned zero external-cache hits.

This PR fixes the type mismatch at the ZMQ decode boundary, without changing
the existing RPC protocol:

• 64-character hex strings are restored with bytes.fromhex().
• Other strings are encoded as bytes.
• Values that are already bytes are passed through unchanged.

As a result, all internal lookup paths receive bytes-like block hashes while
the scheduler-side serialization and wire format remain unchanged.

Does this PR introduce any user-facing change?

No public API, configuration, RPC framing, or cache-key format changes. The fix
prevents affected AscendStore lookups from failing and degrading to zero cache
hits.

How was this patch tested?

• Added a LookupKeyServer regression test that feeds a 64-character hex hash
  through the decoded ZMQ request and verifies that lookup_scheduler()
  receives the corresponding 32-byte BlockHash.
• Verified the regression test fails before the production change because the
  worker receives str, and passes after the boundary conversion is added.
• Ran 50 targeted AscendStore unit tests covering the lookup client, lookup
  server, connector, and byte/string key construction paths.
• Ran ruff check, ruff format --check, and git diff --check on the changed
  files.

No NPU hardware is required for this RPC type-conversion fix.

• vLLM version: v0.26.0
• vLLM main: vllm-project/vllm@0351e9a

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a serialization issue in the AscendStore lookup RPC where block hashes were being incorrectly converted to hexadecimal strings. By preserving the original byte representation during transmission, the change prevents potential type errors in internal hash operations and improves serialization efficiency. The update ensures that the internal contract for block hashes is maintained consistently across the client-server boundary.

Highlights

• RPC Serialization Fix: Modified the AscendStore lookup RPC to preserve block hashes as bytes instead of converting them to hexadecimal strings, ensuring compatibility with downstream consumers that expect binary data.
• Code Refactoring: Renamed the server-side variable 'hashes_str' to 'block_hashes' to accurately reflect the data type being processed.
• Regression Testing: Added a new assertion in the unit tests to verify that the LookupKeyClient correctly passes raw byte hashes to the encoder.

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[gemini-code-assist]

Code Review

Suggested PR Title:

[Ops][Misc] Simplify block hash encoding in pool scheduler

Suggested PR Summary:

### What this PR does / why we need it?
This PR simplifies the block hash encoding process in the pool scheduler by directly encoding raw block hashes instead of converting them to hex strings first. It also renames variables for consistency and updates the unit tests to verify the encoding calls.

### Does this PR introduce _any_ user-facing change?
No.

### How was this patch tested?
Updated unit tests in `test_pool_scheduler.py` to assert the correct arguments are passed to the encoder.

I have no additional feedback to provide as there are no review comments.

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[jichaofdu]

Updated the implementation to normalize decoded hex hashes at the LookupKeyServer boundary. The previous client-side serialization change has been fully removed; the existing RPC wire format is unchanged.