feat(mcp): Add support for filtering on reference_value and timestamp_value in firestore_query_collection

[MrAlek]

Description

Adds the ability to filter on reference values and timestamps in the firestore MCP.

Scenarios Tested

Tested through MCP Inspector.

Sample Commands

  {
    "collection_path": "posts",
    "filters": [
      {
        "field": "author",
        "op": "EQUAL",
        "compare_value": { "reference_value": "users/abc123" }
      }
    ]
  }

  {
    "collection_path": "posts",
    "filters": [
      {
        "field": "createTime",
        "op": "EQUAL",
        "compare_value": { "timestamp_value": "2026-05-09T12:34:56Z" }
      }
    ]
  }

[gemini-code-assist]

Code Review

This pull request enhances the Firestore tool by adding support for reference_value and timestamp_value filters in the query_collection tool, including automatic expansion of relative document paths to full resource names. Feedback highlights a critical bug in the filter validation logic that could lead to runtime crashes or incorrect filtering when multiple values are provided. Additionally, a violation of the repository style guide was noted regarding the use of generic errors instead of FirebaseError for user-facing exceptions.