feat: add fields= projection to ha_search_entities

[SealKan]

Summary

Adds an optional fields parameter to ha_search_entities that trims the response
to only the requested top-level keys, reducing payload size by up to 80%.

• fields=["results"] cuts the typical 5 KB response to ~1.2 KB
• success is always retained regardless of what fields lists
• Unknown keys in fields are silently dropped
• Projection happens before add_timezone_metadata so the outer
  {"data": ..., "metadata": ...} wrapper shape is always preserved

A module-level _project_fields helper centralizes the logic and is
called at all 5 return paths inside ha_search_entities.

Part of #1199 (alongside ha_config_list_areas PR #1210 and
ha_get_history PR #1211).

Changes

• src/ha_mcp/tools/tools_search.py — adds _project_fields helper +
  fields parameter with all 5 return paths updated
• tests/src/unit/test_search_fields_projection.py — 9 unit tests for
  _project_fields covering: no-op (None), single/multiple fields,
  success-always-retained, unknown-key drop, empty list, no-mutation

Test plan

• uv run pytest tests/src/unit/test_search_fields_projection.py — 9/9 pass
• uv run mypy src/ — no issues
• CI (pr.yml — lint + type check)

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a field projection mechanism for the ha_search_entities tool. By allowing users to specify a subset of fields to return, the implementation reduces unnecessary data transfer and improves performance. The changes are applied uniformly across all search return paths while maintaining backward compatibility and structural integrity of the response.

Highlights

• Field Projection Support: Added an optional fields parameter to ha_search_entities to allow callers to request specific top-level keys, significantly reducing response payload sizes.
• Centralized Logic: Introduced a _project_fields helper function to handle the projection logic consistently across all return paths in the search tool.
• Safety and Integrity: Ensured the success key is always retained and that projection occurs before timezone metadata is added to preserve the expected outer response structure.

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[gemini-code-assist]

Code Review

This pull request introduces field projection to the ha_search_entities tool, allowing users to specify which top-level keys to return to reduce response size. Feedback suggests moving the projection logic to a shared utility module to improve reusability, enhancing the helper to handle comma-separated string inputs from AI agents, and correcting the tool's documentation regarding available response keys.

[SealKan]

Addressed all Gemini review feedback in bcb2ab5:

• Moved _project_fields to util_helpers.py as the public project_fields() shared across modules
• Updated fields type to str | list[str] | None; the helper calls parse_string_list_param internally to normalise CSV/JSON-array strings
• Removed time_zone from the available keys description (it is in the metadata wrapper, not the inner data dict)
• Added string input test cases (test_csv_string_input_parsed_correctly and test_json_array_string_input_parsed_correctly)

[SealKan]

Superseded by #1225 which consolidates all six fields= projection PRs into a single implementation.