[core] host_pipeline 'body as T' throws raw TypeError on response-type mismatch

[cemal-yilmaz-bt]

Summary

HostPipeline returns the response with body: parsedBody as T, a runtime cast. When the server's actual body type doesn't match the caller's declared T, this throws an unhandled TypeError instead of returning a handled response/error.

File: packages/dart/morph_core/lib/src/http/host_pipeline.dart:223

Root cause

• parsedBody is typed Object? (L184) and JSON responses are decoded into Map/List (L192, jsonDecode).
• The success path returns body: parsedBody as T (L223).
• If a consumer declares MorphResponse<String> but the endpoint returns content-type: application/json (parsed to a Map), or declares MorphResponse<Map> but the body is a bare JSON array or a non-JSON String, the cast throws a raw TypeError.

Why this is a Dart-specific regression

In the TypeScript source parsed as T is a compile-time-only assertion and never throws at runtime. The Dart as T is checked at runtime, so a response the TS SDK handled fine now crashes.

Repro

A 200 endpoint that a consumer typed as MorphResponse<String> returns content-type: application/json with {"x":1} → crash with an opaque cast error rather than a MorphResponse/MorphHttpError.

Proposed fix

Guard the cast (e.g. is T check → surface a typed decode error or a safe fallback), or return a dynamic/Object? body and let callers coerce, or emit a MorphHttpError/decode error on type mismatch instead of an unguarded cast.

Severity

High — crashes on an otherwise-valid server response.

Found in a local review of PR #37 (head fdbce85).