[Menci]

Restate recognized structured upstream errors into the client's protocol before and during streams. Preserve nullable named fields, unconsumed aliases, envelope metadata and protocol-specific diagnostics while retaining semantic response headers. Unexpected parser failures preserve their error chain, and unrecognized bodies pass through.

Preserve distinct colliding metadata values under the first unused _1, _2, ... suffix. Existing container/inner-error values retain their original key; equal values at the same original key coalesce. Reserve every explicitly stated key before allocating suffixes, and merge namespaces once so source projection cannot duplicate metadata.

Carry Responses continuation failures through the normal event lifecycle. A structured non-streaming error body becomes the corresponding error event, followed by the aggregate response.failed resource. Streaming clients receive both events; non-streaming collection drains the failed resource and renders the original standard error fields as JSON. The original HTTP status and headers are not retained across this event boundary; the downstream HTTP status is reconstructed from the standard error code or classification type, with 502 reserved for unknown classifications. Accumulated output usage is still recorded once.

Protocol errors are JSON; this change adds no plaintext interpretation or status-derived fallback error code. Gemini HTTP 502 errors use the shared protocol mapping and therefore report RPC status UNAVAILABLE.

Depends on #709's prepared-trip contract and local gateway error handling. Focused validation covers flat and nested Responses errors, nullable codes, HTTP classification, extension fields, raw numeric tokens, error-to-failed lifecycle consumers, accumulated usage telemetry, Gemini status mapping, metadata collisions and explicit suffix names. The latest local verification passed 442 named gateway/protocol tests, both owning TypeScript projects and exact changed-file lint.