SDK: sanitize malformed-success HTTP decoding causes

Problem

The TypeScript SDK's shared HTTP transport wraps malformed successful HTTP and SSE responses in ProtocolError, but currently retains the native JSON or protobuf decoder exception as ProtocolError.cause.

JavaScript decoder messages are runtime-dependent. Some engines may include excerpts of the rejected response in an exception message. If a malformed successful response contains sensitive material—such as a short-lived presentation URL—and an application logger captures nested error causes, that material could be copied into application logs.

This is defense-in-depth, not a demonstrated credential disclosure:

• the SDK does not automatically log these errors;
• MecatlError.toJSON() already omits cause;
• the observed reproduction exposed only a URL prefix, not a complete credential; and
• this concerns existing shared transport behavior rather than one server endpoint.

Raised from the non-blocking review follow-up on #1690: https://github.com/stacklok/mecatl/pull/1690#pullrequestreview-5245634245

Direction

Define one shared policy for malformed successful-response decoding at the SDK HTTP boundary:

• unary invalid-JSON and protobuf-decoding failures return the existing generic ProtocolError without the raw decoder cause;
• SSE invalid-JSON and protobuf-decoding failures follow the same rule;
• retain safe metadata such as code, transport, HTTP status, and request ID where available; and
• do not attempt best-effort string redaction of arbitrary decoder messages.

Keep this narrowly scoped. Do not change:

• non-2xx RFC 9457/server-error normalization;
• fetch/network TransportError causes;
• authentication errors;
• abort and cancellation behavior; or
• stream/control lifecycle semantics.

Acceptance criteria

• A unary successful response with malformed JSON rejects with ProtocolError, transport: "http", the HTTP status, no raw cause, and no planted response canary in its message or toJSON() result.
• A unary successful response that parses as JSON but fails descriptor/protobuf decoding has the same cause-free behavior.
• Malformed JSON and descriptor/protobuf failures in ordinary SSE data frames have the same cause-free behavior without changing streaming or cancellation semantics.
• Existing non-2xx server errors retain their typed code, request ID, status, and established cause behavior.
• Existing fetch/network and abort/cancellation tests prove their semantics are unchanged.
• The SDK error documentation and living design notes describe the malformed-success cause policy.

Compatibility

No public type or method signature changes. This intentionally narrows diagnostic detail available through ProtocolError.cause for malformed successful HTTP/SSE payloads so untrusted response content cannot cross into application logging through decoder exceptions.