Add statement-level query_tags support for SEA backend

[sreekanth-db]

Summary

The query_tags feature was previously implemented only for the Thrift backend. Connection-level query tags (via session_configuration["QUERY_TAGS"]) already worked for SEA since it goes through _filter_session_configuration. This PR adds the missing statement-level (per-execute) query tags support for the SEA backend.

Root cause

SeaDatabricksClient.execute_command() accepted the query_tags parameter but had a # TODO comment and silently ignored it — the tags were never included in the ExecuteStatementRequest.

Wire format

The SEA REST API expects query_tags as an array of {key, value} objects, unlike Thrift which uses a serialized string in confOverlay:

"query_tags": [
  {"key": "team", "value": "data-eng"},
  {"key": "experimental"}
]

• None values are passed through as-is — server handles them as key-only tags
• Empty or None dict omits the field entirely from the request
• No client-side escaping needed — the {key, value} JSON format handles special characters natively (unlike the serialized string format used in Thrift/session path)

Changes

• src/databricks/sql/backend/sea/models/requests.py: Add query_tags field to ExecuteStatementRequest dataclass; serialize to [{key, value}] array in to_dict()
• src/databricks/sql/backend/sea/backend.py: Pass query_tags to ExecuteStatementRequest constructor; remove TODO comment

Tests

Unit tests added (tests/unit/test_sea_backend.py)

• test_execute_command_query_tags_string_values — string values serialized correctly to [{key, value}]
• test_execute_command_query_tags_none_value — None values passed through as {"key": k, "value": None}
• test_execute_command_no_query_tags_omitted — field absent when query_tags=None
• test_execute_command_empty_query_tags_omitted — field absent when query_tags={}
• test_execute_command_async_query_tags — tags included in async (execute_async) path

CI checks verified locally

poetry run black --check src           ✓ all files unchanged
poetry run mypy --non-interactive src  ✓ no issues in 59 source files
poetry run python -m pytest tests/unit ✓ all passing

E2E test

Ran examples/query_tags_example.py with use_sea=True against a live Databricks SQL warehouse. All 4 examples passed — connection-level tags, per-query tags (including None values and special characters), async execution, and executemany. Verified tags stored correctly in query history UI including special characters (colons, commas, backslashes) — no escaping needed for the JSON {key, value} format.