[CI/Build] Fix compile warnings

[Aionw]

Description

Fix compile warnings in Mooncake-owned C++ code without changing compiler flags.

This addresses:

• pybind11 visibility warnings in the RPC communicator by hiding pybind11 callback implementation details behind a holder type.
• unused-variable warnings by preserving the variables and marking them [[maybe_unused]] where they are intentionally retained for metrics/debug context.
• MasterService member initialization order warnings by matching declaration order.
• a deprecated volatile compound assignment in mmap_arena_test by using an explicit assignment.

Checked existing open PRs/issues for related compile-warning work. The only open PR found by the search was #2540, which is about MUSA EP builds and does not overlap this change.

Module

• Transfer Engine (mooncake-transfer-engine)
• Mooncake Store (mooncake-store)
• Mooncake EP (mooncake-ep)
• Mooncake PG (mooncake-pg)
• Integration (mooncake-integration)
• P2P Store (mooncake-p2p-store)
• Python Wheel (mooncake-wheel)
• Common (mooncake-common)
• Mooncake RL (mooncake-rl)
• CI/CD
• Docs
• Other

Type of Change

• Bug fix
• New feature
• Refactor
• Breaking change
• Documentation update
• Performance improvement
• Other

How Has This Been Tested?

Test commands:

cmake -S . -B build-codex -G Ninja -DWITH_STORE_RUST=OFF
cmake --build build-codex -j8
git diff --check
pre-commit run --files mooncake-store/src/file_storage.cpp mooncake-store/src/master_metric_manager.cpp mooncake-store/src/master_service.cpp mooncake-store/src/real_client.cpp mooncake-store/tests/mmap_arena_test.cpp mooncake-transfer-engine/include/transport/rpc_communicator/rpc_communicator.h mooncake-transfer-engine/src/transport/rpc_communicator/rpc_communicator.cpp mooncake-transfer-engine/src/transport/rpc_communicator/rpc_interface.cpp

Test results:

• Unit tests pass
• Integration tests pass (if applicable)
• Manual testing done (describe below)

Build completed successfully. The only remaining warning observed during configuration was from extern/pybind11, which is dependency code.

pre-commit run --files passed the standard hooks, but the project format hook scans a wider diff against origin/main and repeatedly rewrote an unrelated file (mooncake-store/include/kv_event/kv_event_config.h). That unrelated formatting change was restored and not included in this PR.

Checklist

• I have performed a self-review of my own code
• I have formatted my code using ./scripts/code_format.sh
• I have run pre-commit run --all-files and all hooks pass
• I have updated the documentation (if applicable)
• I have added tests to prove my changes are effective
• For changes >500 LOC: I have filed an RFC issue

No documentation or new tests are needed; this change only removes compile warnings from existing code paths.

AI Assistance Disclosure

• No AI tools were used
• AI tools were used (specify below)

Codex was used to identify warning sources, apply the source changes, run build/pre-commit validation, and prepare this PR.

[gemini-code-assist]

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[codecov-commenter]

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Codecov Report

❌ Patch coverage is 57.89474% with 8 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...rc/transport/rpc_communicator/rpc_communicator.cpp	0.00%	4 Missing ⚠️
mooncake-store/tests/mmap_arena_test.cpp	0.00%	3 Missing ⚠️
mooncake-store/src/file_storage.cpp	0.00%	1 Missing ⚠️

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