feat(store): add batch exist support for master

[xiaguan]

This change is in preparation for future optimizations like LMCache/LMCache#891. The LMCache scheduler often checks the existence of many keys at once, and using this new interface can significantly reduce latency for those lookups.

Upcoming TODO:

Add support for meta-only clients (which don't need to interact with the actual data).

[Copilot]

Pull Request Overview

This PR adds support for batch existence checks for keys in both server and client code (including a Python interface), which will help reduce latency in high-volume key lookup scenarios. Key changes include:

• Introducing new methods BatchExistKey/BatchIsExist in MasterService, MasterClient, Client, and their respective tests.
• Updating related RPC and Python binding interfaces to accommodate batch exist operations.
• Formatting changes and minor improvements, such as additional key parameter validation.

Reviewed Changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
mooncake-wheel/tests/test_distributed_object_store.py	Added test for batch is_exist operations.
mooncake-store/tests/master_service_test.cpp	New tests for BatchExistKey and formatting updates.
mooncake-store/tests/client_integration_test.cpp	Added and updated tests for batch is_exist operations.
mooncake-store/src/master_service.cpp	Added BatchExistKey and enhanced PutStart validation.
mooncake-store/src/master_client.cpp	Implemented BatchExistKey with proper error handling and logging.
mooncake-store/src/client.cpp	Added BatchIsExist method with duplicate-check validation.
mooncake-store/include/rpc_service.h	Introduced new structure and handler for BatchExistKey.
mooncake-store/include/master_service.h, master_client.h, client.h	Updated headers with new batch exist API declarations.
mooncake-integration/store/store_py.h, store_py.cpp	Added Python binding for batch exist functionality.

Comments suppressed due to low confidence (1)

mooncake-store/include/rpc_service.h:100

• The structure name 'BatchExistReponse' appears to be misspelled. Consider renaming it to 'BatchExistResponse' for improved clarity and consistency with common naming conventions.

struct BatchExistReponse {

[ykwd]

Great work. This pr looks good to me. By the way, since there are many API changes recently, are our docs already obsolete?

[xinranwang17]

I have implemented a similar interface here: #511
The difference is that the return value is a map of <key, is_exist> pair, which is easy for upper level caller to find the existed key without addressing them by index again.
The current sglang l3 cache is using map of <key, is_exist> pair as the return value, it seems LMcache use the different one. Should we have an agreement before merging all related prs.