[copybara-service]

fix: surface backend error messages in Java and Go Interactions SDK errors

Since openapi-generation v2.960.7, Speakeasy fills the error message from error.message only for operations with a typed error schema (the Interactions *ClientError / *ServerError carriers in Java and Go). The untyped fallback (GaosApiException in Java, APIError in Go), used by operations without a typed error schema and by bodies that do not match it (e.g. the [{error: {...}}] arrays streaming endpoints return), still passes the placeholder "API error occurred" and leaves the backend's {error: {message}} only in the raw response body. So a 400 validation error from such an operation surfaces in Java as "400 . API error occurred" and in Go as APIError.Message == "API error occurred", and no Java gaos error populates ApiException.status() or carries error.details, so even typed errors read "400 . <message>".

• Java: Emits package-private GaosErrorBody (scripts/templates/error_body.java.tmpl). GaosBaseException, GaosClientException, and GaosServerException forward GaosErrorBody.status(body) (OnePlatform error.status, else the Interactions API string error.code, else "") and GaosErrorBody.message(message, body) to com.google.genai.errors.ApiException. The message is error.message plus any error.details, appended one per line after \nDetails: ; for {error: {...}} bodies this matches the native ApiException.getErrorMessageFromBody, so gaos and native errors read the same.
• Go: Emits messageFromBody (scripts/templates/error_body.go.tmpl) in interactions/models/apierrors/errorbody.go and calls it from NewAPIError, which fills APIError.Message from error.message.
• Both read the {error: {...}} object and the single-element [{error: {...}}] array that streaming endpoints return, replace only the placeholder (a more specific message is kept), and keep the placeholder when the body is not a recognized JSON error (including multi-element arrays).
• Behavior change (Java): getMessage() now spans several lines for errors that carry error.details (e.g. 400 INVALID_ARGUMENT. Bad thing\nDetails: {"@type":"type.googleapis.com/google.rpc.ErrorInfo","reason":"API_KEY_INVALID"}), so callers that log or compare the text see the extra line. message() gains the same lines: on the per-operation *ClientError / *ServerError carriers it is the message text alone (Bad thing\nDetails: …), as on the native ApiException, while GaosApiException keeps Speakeasy's existing override that returns the full getMessage() string. Go's APIError.Message stays error.message only; details remain in the body that Error() appends, following each language's native convention.
• Documented differences from the native Java ApiException: status() falls back to the Interactions error.code where native uses the HTTP reason phrase (empty on HTTP/2); a single-element array also yields its details and an empty missing status where native throwFromErrorNode drops details and reports UNKNOWN; a multi-element array keeps the placeholder where native reads the first element. These are stated in the GaosErrorBody Javadoc and pinned by tests.
• Sync hardening & tests: reparent_java_error replacements in sync_speakeasy_outputs.py now use _replace_once, so generator template changes fail loudly at sync time; adds patch_go_apierrors; adds Java (including parity and divergence tests against the native ApiException), Go, and sync unit tests.