feat: implement unified options system

[weinong]

• Introduce unified options architecture for both convert and token commands aiming to simplify options-to-cli flag mapping, validation, and conversion
• The unified options are enabled by default. Set environment variable KUBELOGIN_USE_LEGACY_OPTIONS=true to fallback to previous implementation
• Add comprehensive validation with strict/lenient modes for different contexts
• Implement reflection-based argument building for kubeconfig conversion
• Add extensive test coverage for unified options and validation
• Establish breadcrumb documentation system for development decisions

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 61.50289% with 333 lines in your changes missing coverage. Please review.

Project coverage is 56.71%. Comparing base (15edb67) to head (72e90f5).
Report is 49 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/internal/options/execution.go	51.44%	183 Missing and 19 partials ⚠️
pkg/internal/options/validation.go	69.47%	53 Missing and 5 partials ⚠️
pkg/internal/options/unified.go	74.03%	41 Missing and 6 partials ⚠️
pkg/internal/options/command.go	63.88%	10 Missing and 3 partials ⚠️
pkg/cmd/convert.go	0.00%	8 Missing ⚠️
pkg/cmd/token.go	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #689      +/-   ##
==========================================
- Coverage   63.43%   56.71%   -6.73%     
==========================================
  Files          37       47      +10     
  Lines        2210     3269    +1059     
==========================================
+ Hits         1402     1854     +452     
- Misses        733     1280     +547     
- Partials       75      135      +60     

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[Copilot]

Pull Request Overview

This pull request implements a unified options system for both convert and token commands aiming to simplify options-to-cli flag mapping, validation, and conversion. The unified options are enabled by default with environment variable KUBELOGIN_USE_LEGACY_OPTIONS=true to fallback to previous implementation.

Key changes include:

• Introduction of unified options architecture with automatic flag registration using reflection and struct tags
• Implementation of comprehensive validation with strict/lenient modes for different contexts
• Addition of reflection-based argument building for kubeconfig conversion with field extraction from existing configurations

Reviewed Changes

Copilot reviewed 31 out of 33 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
test/integration/test_helpers.go	New integration test framework with environment isolation and helper utilities
test/integration/kubeconfig_flag_test.go	Tests for --kubeconfig flag support in unified mode
test/integration/go.mod	Go module configuration for integration tests
test/integration/fixtures/sample_kubeconfig.yaml	Sample kubeconfig fixture for testing conversion scenarios
test/integration/fixtures/empty_kubeconfig.yaml	Basic kubeconfig fixture for testing minimal configurations
test/integration/converter_integration_test.go	Comprehensive integration tests for all login methods and conversion scenarios
test/integration/cli_integration_test.go	CLI argument parsing and environment variable integration tests
pkg/internal/options/validation_test.go	Unit tests for validation rules and login method validation
pkg/internal/options/validation.go	Validation logic with strict and lenient modes for different command contexts
pkg/internal/options/utils.go	Utility functions including ToString() with sensitive field masking
pkg/internal/options/unified_test.go	Unit tests for unified options core functionality
pkg/internal/options/unified.go	Core unified options struct with automatic flag registration and environment loading
pkg/internal/options/kubeconfig_test.go	Tests for kubeconfig flag registration and parsing
pkg/internal/options/field_extraction_diagnostic_test.go	Diagnostic tests demonstrating field extraction validation issues
pkg/internal/options/execution_test.go	Tests for command execution and options conversion
pkg/internal/options/execution.go	Command execution logic with field extraction and kubeconfig conversion
pkg/internal/options/description_consistency_test.go	Tests ensuring consistent flag descriptions between legacy and unified modes
pkg/internal/options/convert_test.go	Unit tests for conversion logic and exec config building
pkg/internal/options/command_test.go	Tests for unified command creation and feature flag behavior
pkg/internal/options/command.go	Command creation utilities and feature flag management
pkg/cmd/token.go	Integration of unified options in token command with legacy fallback
pkg/cmd/convert.go	Integration of unified options in convert command with legacy fallback
Makefile	Added integration test targets and output management
.github/instructions/options-and-conversion.instructions.md	Comprehensive documentation for unified options system usage
.github/instructions/go.instructions.md	Go development best practices and coding standards
.github/copilot-instructions.md	Development workflow instructions and breadcrumb protocol
.github/.copilot/breadcrumbs/2025-07-20-1430-flag-description-consistency.md	Documentation of flag description consistency improvements
.github/.copilot/breadcrumbs/2025-07-20-1405-extract-login-constants.md	Documentation of login constants consolidation refactoring