feat: expose HTTP status code and headers on SDK error classes

[corvid-agent]

Summary

• Adds optional statusCode and headers properties to all A2A SDK error classes (TaskNotFoundError, TaskNotCancelableError, PushNotificationNotSupportedError, UnsupportedOperationError, ContentTypeNotSupportedError, InvalidAgentResponseError, AuthenticatedExtendedCardNotConfiguredError) so that consumers can implement HTTP-aware error handling.
• Updates the REST transport (RestTransport) to extract HTTP status code and response headers from the Response object and pass them through when constructing error instances via mapToError.
• Exports the new A2AErrorHttpDetails interface from the client entry point for use by consumers.
• Adds comprehensive tests covering all error types, verifying that statusCode and headers are correctly populated on errors thrown by the REST transport.

Closes #317

Test plan

• All 414 existing tests pass (no regressions)
• New tests verify statusCode is set on TaskNotFoundError (404), TaskNotCancelableError (409), PushNotificationNotSupportedError (400), UnsupportedOperationError (405), ContentTypeNotSupportedError (415), InvalidAgentResponseError (502), and AuthenticatedExtendedCardNotConfiguredError (401)
• New test verifies that response headers (e.g. Retry-After, X-RateLimit-Remaining) are available on errors for rate-limiting / backoff strategies
• Backward compatibility verified: constructor changes are additive (second parameter is optional), so existing code including JSON-RPC error subclasses continues to work without modification
• TypeScript lint passes with no new errors

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