fix(index): Fixing name collision for label tags in tantivy index (#2219)

[srivik]

Description:

Summary

• Fixes a bug where filtering by a label whose name matches the MapColumn name
  (e.g., tags="b1.metal" when the partition schema MapColumn is also named "tags")
  returns no data in the Tantivy index
• The root cause is Tantivy's find_field_with_default resolving the field name to
  the JSON field itself with an empty prefix, producing an incorrect query term
• Group-by (by (tags)) was unaffected because it reads from partition key binary data
  directly, bypassing the index

Changes

• Added resolve_field_with_default() in query_parser.rs that detects when a field
  name collides with the JSON column name and treats it as a subfield path
• Applied the fix to equals, regex, prefix, and term-in query paths
• Applied equivalent fix in reader.rs for label value lookups

Test plan

• Rust unit tests: equals, regex, term-in, prefix with map column name collision
• Scala integration test: end-to-end indexing + querying with tags:map schema
• Verify on deployment with part-key-index-type = tantivy that tags="value"
  filters now return correct results