feat(client): expose validation result in hooks

[romanindev]

Summary

Exposes response validation metadata in lifecycle hooks.

Changes

• add ResponseValidationResult type
• store validation result in the execution context
• expose validation metadata in afterResponse
• add integration coverage for validation hook metadata

Behavior

When response validation is configured and passes, afterResponse receives:

validation: {
  enabled: true,
  passed: true
}

When validation is not configured, the validation field is omitted.

Closes #57