feat: Add HeadlessRepository implementation

[bthebladeprimer]

Summary

• Add HeadlessRepositoryImpl — the central data repository bridging CheckoutComponents to the existing Headless SDK infrastructure
• Add comprehensive test suite covering: analytics, delegate handling, payment methods retrieval, payment flow, card processing, raw data manager delegation, card network selection, validation, and vaulted payments
• Add ContainerTests verifying DI container wiring
• Add HeadlessRepositorySettingsTests for configuration scenarios

Context

PR 11 of 16 in the CheckoutComponents feature split. Depends on PR 10 (core interactors, mapper & bridge).

Notion tracker

Test plan

• HeadlessRepositoryGetPaymentMethodsTests — verify payment method fetching
• HeadlessRepositoryProcessCardPaymentTests — verify card payment processing
• HeadlessRepositoryPaymentFlowTests — verify end-to-end payment flow
• HeadlessRepositoryDelegateTests — verify delegate callback handling
• HeadlessRepositoryAnalyticsTests — verify analytics event tracking
• HeadlessRepositoryValidationFlowTests — verify input validation
• HeadlessRepositorySelectCardNetworkTests — verify card network selection
• HeadlessRepositoryVaultTests / VaultMockTests — verify vaulted payment handling
• HeadlessRepositoryRawDataManagerDelegateTests — verify raw data manager delegation
• HeadlessRepositoryOneShotContinuationTests — verify one-shot continuation pattern
• HeadlessRepositoryAdditionalCoverageTests — edge case coverage
• HeadlessRepositorySettingsTests — verify settings/configuration
• ContainerTests — verify DI container registration

[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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