feat: support parent field in work items

[MR implemented by Duo]

Relates to issue #74 (closed)

Changes

This MR implements complete parent field support for work items in Rust, backporting functionality from the TypeScript implementation in MR !276 (merged).

Key Changes

Type System

• Added Parent field variant to the Field enum
• Added WorkItemRef reference type with "#" symbol for issue references
• Implemented string mappings and display implementations

Parser System

• Created generic work_item() parser function supporting both epic (&) and issue (#) references
• Refactored epic() parser to use the generic function
• Added new issue() parser for work item references
• Integrated issue parser into the value parsing chain
• Added comprehensive parser tests

Utility Functions

• Implemented parse_work_item_reference() function to parse both epic and work item references
• Added comprehensive tests covering epic references, issue references, edge cases, and fallback scenarios

Analyzer Integration

• Added Parent field validation to the Issues source analyzer
• Integrated Parent field type checking using the field mapping system

Field Mapping

• Added parent_work_items_attribute() function for parent field attribute mapping
• Created Parent field mapping configuration supporting both WorkItemRef and EpicRef
• Enhanced transform_epic_subquery_with_variable_type() to handle both Epic and Parent fields
• Implemented logic to generate appropriate subqueries based on reference type (epic vs work item)

GraphQL Filter Generation

• Added Parent field value transformation in to_graphql_filter_value()
• Added Parent field key mapping in to_graphql_filter_key()
• Updated GraphQLFilters constraints to include Parent field for descendant work items

Testing

• Added tests for parent field with epic references
• Added tests for parent field with issue/work item references
• Added tests for parent field in lists (both epic and issue references)
• Added test for parent field without work items feature flag
• Added parser tests for issue references
• Added utility function tests for work item reference parsing

Features

• Generic Parent field works for all work items (tasks, key results, issues)
• Dual reference support for both epic references (&) and work item references (#)
• Smart path resolution automatically derives group/project paths from references
• Feature flag integration (only available when GlqlWorkItems feature flag is enabled)
• Comprehensive subquery generation creates appropriate GraphQL subqueries based on reference type
• Full test coverage for all major use cases and edge cases