feat: Add validation system

[bthebladeprimer]

Summary

• Adds ValidationRule protocol and ValidationResult types for composable field validation
• Adds ValidationService that orchestrates rule evaluation for form fields
• Adds card-specific validation rules (card number, CVV, expiry date, cardholder name)
• Adds common validation rules (email, phone, address, city, state, postal code, country code, OTP)
• Adds RulesFactory for creating validation rule sets per field type
• Adds ExpiryDateInput model for structured expiry date handling
• Includes 14 comprehensive test files covering all validation rules and edge cases

Context

PR 5 of 15 in the CheckoutComponents feature split. The validation system is used by the public API layer (PR 6) and all UI components. Depends on PR 4 (#1633).

Tracker: https://www.notion.so/32fca65dc30e81dabf66f33e2b58c3a1

Test plan

• Verify all 14 validation test files pass
• Confirm card number Luhn check works correctly
• Confirm expiry date edge cases (month boundaries, year rollover) are handled
• Verify RulesFactory returns correct rules for each field type
• Run: xcodebuild -workspace PrimerSDK.xcworkspace -scheme PrimerSDKTests -destination "platform=iOS Simulator,name=iPhone 16" test

[github-actions]

	Warnings
⚠️	> Pull Request size seems relatively large. If this Pull Request contains multiple changes, please split each into separate PR will helps faster, easier review.

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