test(python/schemas): add unit tests for hooks dataclasses

[0xAxiom]

Summary

Add 34 unit tests for python/x402/x402/schemas/hooks.py — the largest source file in the schemas/ package without dedicated unit-test coverage. Mirrors the same coverage pattern as recent unit-test PRs (tests/unit/mechanisms/...) into a new tests/unit/schemas/ directory.

The hooks module defines lifecycle dataclasses used by x402Client, x402ResourceServer, and x402Facilitator. Several context subclasses use __post_init__ validation to enforce that an inherited-with-default field (result, error, payment_payload) is always set explicitly. Those validations had no test coverage; this PR pins them.

What's covered

• AbortResult, RecoveredPayloadResult, RecoveredVerifyResult, RecoveredSettleResult — field storage and equality
• VerifyContext, SettleContext — optional payload_bytes / requirements_bytes default to None and accept explicit bytes
• VerifyResultContext, SettleResultContext, PaymentCreatedContext — happy path stores inherited fields plus the new field, and __post_init__ raises ValueError when the new field is missing or explicitly None
• VerifyFailureContext, SettleFailureContext, PaymentCreationFailureContext — same pattern for the error field
• PaymentCreationContext — payment_required and selected_requirements storage

Tests use unittest.mock.MagicMock for the forward-referenced payload / requirements / response types, keeping the suite free of network and pydantic-validation overhead.

Test plan

• uv run pytest tests/unit/schemas/test_hooks.py -v — 34 passed
• uv run pytest tests/unit — 888 passed (full unit suite)
• uvx ruff format tests/unit/schemas/ — clean
• uvx ruff check tests/unit/schemas/ — All checks passed
• GPG-signed commit

No source-code changes; tests-only. No changelog fragment per python/CONTRIBUTING.md (towncrier fragments are for user-facing changes).

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