Add tests for parsing the Gate field in formula steps:
- TestParse_GateField: JSON parsing with all gate fields
- TestParse_GateFieldTOML: TOML parsing
- TestParse_GateFieldMinimal: minimal gate with only type
- TestParse_GateFieldWithAllTypes: all gate types (gh:run, gh:pr, timer, human, bead, mail)
- TestParse_GateInChildStep: gate in nested child steps
The Gate struct and Step.Gate field already existed. These tests
verify TOML and JSON parsing works correctly.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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