feat(azure): add idle Cosmos DB for Gremlin API check (#20)

[DYNOSuprovo]

Resolves #20

Description

This PR implements the idle Cosmos DB provisioned throughput check for the Gremlin (Graph) API, following the project's standard 4-file check pattern:

1. Source Plugin: src/cloudcost/sources/azure/cosmosdb_gremlin_idle_ru.py (AzureCosmosDbGremlinIdleRuSource)
   • Discovers non-serverless Cosmos DB accounts configured with Gremlin API (EnableGremlin capability).
   • Collects provisioned throughput across both database-level (az cosmosdb gremlin database throughput show) and graph-level (az cosmosdb gremlin graph throughput show) scopes.
   • Fetches consumption metrics via Azure Monitor (TotalRequestUnits over lookback_days).
2. SQL Policy: policies/cosmosdb_gremlin_idle_ru.sql
   • Billed cost calculation based on official Azure Retail Prices for provisioned RU/s ($0.008/hr per 100 RU/s = (provisioned_ru / 100.0) * 0.008 * 730).
   • Flags accounts where provisioned RU > 0 and consumed RU < 1% of provisioned RU.
3. Pipeline Config: cloudcost.cosmosdb-gremlin-idle-real.yml
4. Engine Runner Registration: Registered in src/cloudcost/engine/runner.py.
5. Tests: Added comprehensive policy test suite in tests/policies/test_cosmosdb_gremlin_idle_ru.py (5 tests covering idle accounts, active accounts, zero provisioned RU, mixed tables, and empty tables).
6. Documentation: Updated README.md under Databases & messaging to include Gremlin API.

Verification

• Ran all unit tests locally with pytest and DuckDB:

  uv run pytest tests/ -v
  ============================= 50 passed in 11.59s =============================
  

[raphgm]

Verified: pulled the branch, ran all 50 tests locally (matches the PR's claimed count), all pass. Source file follows the established Cosmos-API pattern exactly (same structure as cosmosdb_mongo_idle_ru.py / cosmosdb_cassandra_idle_ru.py from earlier), correctly uses its own CLI surface (az cosmosdb gremlin ...) and resource scope. Real, well-scoped PR closing #20. Author has solid trust signals (3-year-old account, 159 repos). Approving.