Implement topic resolver and optimize cache loading

[keyurva]

• Adds support for the new resolver = "topic" option inside the /v2/resolve API and a parameter to request expanding topic hierarchies.
• Integrates dynamic, incremental SV metadata caching into the topic cache manager.
• Excludes topics and SVPGs that lack populated list properties in the database.
• Prunes dangling parent-child tree references recursively when child targets are empty or skipped.
• Bypasses Spanner's edge pagination truncation limit globally by parallelizing multi-node requests in chunked partitions.

Verification / Testing Reference

1. Topic Resolver (expand_topics = false)

Returns immediate sub-topic and variable candidates:

curl -X POST -H "Content-Type: application/json" -H "X-Skip-Cache: true" -d '{
  "nodes": ["dc/topic/Root"],
  "resolver": "topic",
  "expand_topics": false
}' http://localhost:8081/v2/resolve | jq

2. Topic Resolver (expand_topics = true)

Recursively expands the entire nested hierarchy cache:

curl -X POST -H "Content-Type: application/json" -H "X-Skip-Cache: true" -d '{
  "nodes": ["dc/topic/Root"],
  "resolver": "topic",
  "expand_topics": true
}' http://localhost:8081/v2/resolve | jq

3. Embeddings/Indicator Resolver (expand_topics = false)

Performs semantic embeddings search and returns immediate candidates:

curl -X POST -H "Content-Type: application/json" -H "X-Skip-Cache: true" -d '{
  "nodes": ["health"],
  "resolver": "indicator",
  "expand_topics": false
}' http://localhost:8081/v2/resolve | jq

4. Embeddings/Indicator Resolver (expand_topics = true)

Performs semantic search and recursively expands matched topic hierarchies:

curl -X POST -H "Content-Type: application/json" -H "X-Skip-Cache: true" -d '{
  "nodes": ["health"],
  "resolver": "indicator",
  "expand_topics": true
}' http://localhost:8081/v2/resolve | jq

[gemini-code-assist]

Code Review

This pull request introduces a new topic resolver for explicit topic tree navigation and enhances the indicator resolver to support recursive topic expansion. Key changes include the implementation of a TopicExpander interface, a read-through cache for Statistical Variable metadata in TopicCacheManager, and parallelized node fetching in the datasource layer to improve performance and avoid database limits. Feedback focuses on optimizing the cache implementation to prevent stampedes, improving recursive slice allocations, and ensuring robust error handling and logging across the new topic expansion logic.