feat(api): Add thread_id to SearchFilter

[mmaudet]

Summary

Add thread_id field to SearchFilter to allow searching messages by thread ID via the search API.

This enables users to find all messages belonging to a specific conversation thread, complementing the existing get-thread-messages endpoint.

Changes

• Add thread_id: Option<u64> to SearchFilter struct in src/modules/message/search.rs
• Add thread_id query handling in filter_query function in src/modules/indexer/manager.rs

Example Usage

POST /api/v1/search-messages
{
  "filter": {
    "account_id": 5226761534500230,
    "thread_id": 8416787662419872
  },
  "page": 1,
  "page_size": 10
}

Test plan

• Verify SearchFilter schema includes thread_id in OpenAPI spec
• Test search by thread_id returns all messages in the thread
• Verify existing search functionality still works

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