[Store] Clarify cache stats semantics - #2248

Description

Refs #972.

This PR clarifies the semantics of Mooncake Store cache stats.

The existing *_HIT_RATE values in calculate_cache_stats() are Store-observed reuse signals. They divide cumulative Store-side hits by the current cached object count, so they may exceed 1.0. They are not end-to-end request-level or token-level cache hit ratios.

This change:

• Adds clearer enum aliases such as *_HITS_PER_CURRENT_CACHED_OBJECT and *_CURRENT_CACHED_OBJECTS.
• Keeps the existing *_HIT_RATE, *_TOTAL, and VALID_GET_RATE enum values for RPC/API compatibility.
• Updates comments and metric descriptions to avoid presenting Store-side stats as final request/token-level hit ratios.
• Documents that request-level hit ratio should be calculated by Conductor or the inference engine.
• Adds test coverage for enum value compatibility, alias equivalence, isolated CalcCacheStatsTest execution, and values greater than 1.0.
• Adds the missing transport-selector TENT toctree entry so the strict Sphinx docs build can include the existing page.

Module

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

Type of Change

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

How Has This Been Tested?

On a remote Linux validation host:

• git diff --check HEAD~1
• ./scripts/code_format.sh --check --base HEAD~1
• cd docs && make html SPHINXOPTS=-W SPHINXBUILD=/root/miniconda3/bin/sphinx-build
• cmake --build build --target master_metrics_test -j"$(nproc)"
• ./build/mooncake-store/tests/master_metrics_test --gtest_filter=MasterMetricsTest.CalcCacheStatsTest
• MC_METADATA_SERVER=http://127.0.0.1:8080/metadata DEFAULT_KV_LEASE_TTL=500 ctest --test-dir build -R master_metrics_test --output-on-failure

Checklist

• I have performed a self-review of my own code.
• I have formatted my own code using ./scripts/code_format.sh before submitting.
• I have updated the documentation.
• I have added tests to prove my changes are effective.