test: add sync-validation tests for config strings and translations

[florianhorner]

Summary

• Adds tests/test_strings_sync.py with 6 tests that prevent drift between strings.json, en.json, services.yaml, and translation files
• Tests use only stdlib + yaml (no homeassistant dependency) so they can run without the HA test environment
• Validates: strings.json ↔ en.json structure, translation file step/error key coverage, services.yaml field documentation, error key completeness

Tests

1. test_strings_and_en_json_step_keys_match — step names match between strings.json and en.json
2. test_translation_files_have_step_structure — all 38 translation files define the same step keys
3. test_translation_files_have_error_keys — all translation files define the same error keys
4. test_services_yaml_fields_have_descriptions — every service field has a description
5. test_error_keys_cover_config_flow — error keys in strings.json cover what config_flow.py uses
6. test_en_json_matches_strings_json_options — en.json and strings.json options are structurally identical

Motivation

Currently there's no automated check that UI strings, service descriptions, and translations stay in sync. These tests catch missing keys, structural drift, and undocumented fields before they ship.

Test plan

• Ruff lint passes
• Syntax validation passes
• CI pytest run

🤖 Generated with Claude Code