test: enhance unit test coverage across internal packages

[mskutin]

Summary

Improve test coverage for three packages with low coverage: internal/budgets, internal/cmd, and internal/policy.

Test Coverage Improvements

Package	Before	After	Improvement
internal/budgets	51.5%	70.7%	+19.2%
internal/cmd	14.7%	28.7%	+14.0%
internal/policy	44.0%	76.0%	+32.0%

Changes

internal/budgets/client_test.go (+40 test cases, ~736 lines)

• Tests for NewClientWithAssumeRole() - Role assumption constructor
• Tests for getClientForAccount() - Core role assumption logic
• Tests for parseBudgetConfig() - AWS Budget to internal format conversion
• Tests for GetAllAccountsBudgetsWithProgress() - Progress callback functionality
• Role assumption scenarios (valid ARN, invalid account, non-existent role)
• Budget parsing with various configurations (valid, missing fields, invalid limits)
• Concurrent access patterns
• Error classification for AWS API errors

internal/cmd/root_test.go (+73 test cases, ~904 lines)

• Tests for printBanner() - Banner printing with version info
• Tests for loadAWSConfig() - AWS SDK configuration loading
• Tests for filterAccounts() - Account filtering by ID
• Tests for filterAccountsByOU() - OU filtering
• Tests for discoverAccounts() - AWS Organizations account discovery
• Account filtering scenarios
• Flag availability and default values validation
• Context cancellation handling

internal/policy/resolver_test.go (+72 test cases, ~1,644 lines)

• Tests for LoadAccountMetadata() - AWS API calls for OU and tag info
• Tests for ValidateOUs() - OU validation against AWS Organizations
• Successful OU and tag loading scenarios
• Permission error handling (graceful degradation)
• Policy resolution priority rules (account > tag > OU > default)
• Complete override scenarios
• Zero value override handling

Testing

go test ./internal/budgets -v -cover   # 70.7% coverage
go test ./internal/cmd -v -cover       # 28.7% coverage
go test ./internal/policy -v -cover    # 76.0% coverage
go test ./... -cover                    # all packages

Checklist

- Tests follow table-driven test pattern where applicable
- Tests cover both success and error paths
- Tests include edge cases (empty results, nil pointers, invalid formats)
- All tests pass
- Code formatted with gofmt

[codecov-commenter]

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