Add ResolvePlaces API, tabular place_dcids, and Spanner topic expansion

[keyurva]

• Added ResolvePlaces endpoint in the agent service to resolve place names to DCIDs with 1:1 row alignment (including placeholder rows for unresolved queries).
• Updated SearchIndicators to support tabular place_dcids input, preserve input place ordering, and aggregate child topic place availability prior to topic pruning.
• Updated SpannerDataSource vector search resolution to expand topic candidates when expand_topics: true is set.
• Added table-driven unit tests for place resolution edge cases and place existence filtering.

Verification

Tested locally as follows:

1. ResolvePlaces Alignment Test (valid and unresolved places):

curl -X POST http://localhost:8081/v2/agent/resolve_places \
  -H "Content-Type: application/json" \
  -d '{
    "queries": ["california", "nonexistent_place_xyz"]
  }' | jq .

2. SearchIndicators with Tabular place_dcids & Topic Expansion:

curl -X POST http://localhost:8081/v2/agent/search_indicators \
  -H "Content-Type: application/json" \
  -d '{
    "query": "health",
    "place_dcids": ["geoId/06"],
    "per_search_limit": 5,
    "expand_topics": true,
    "include_topics": true
  }' | jq .

[gemini-code-assist]

Code Review

This pull request introduces a new V2AgentResolvePlaces RPC endpoint to resolve place name queries to canonical DCIDs, names, and types. It also refactors the SearchIndicators endpoint to support a tabular response format and accept explicit place_dcids directly, bypassing place resolution. Additionally, common helper functions and constants are refactored into a new util.go file. The review feedback suggests using the generated protobuf getter GetMetadata() instead of accessing the Metadata field directly in search_indicators.go for safer and more idiomatic code.