Improve error propagation where errors were silently swallowed

[devin-ai-integration]

Summary

Several code paths discarded errors with _ = or err == nil guards that fell through silently, hiding real failures (corrupt writes, bad config, non-serializable responses). This tightens those spots so errors are either propagated or logged, without changing normal-path behavior.

Changes:

• codex.writeAuthJSON — the file Close() error was dropped via defer f.Close(), so a flush-on-close failure could report success while auth.json was truncated. Now uses a named return that captures the close error:

  func writeAuthJSON(path, token string) (err error) {
      ...
      defer func() {
          if closeErr := f.Close(); closeErr != nil && err == nil {
              err = fmt.Errorf("close %s: %w", path, closeErr)
          }
      }()
      if err := json.NewEncoder(f).Encode(...); err != nil { return ... }
      return nil
  }

• server.writeSSE — payload, _ = json.Marshal(event.Data) swallowed marshal failures and emitted a truncated/nil data: frame. Now returns fmt.Errorf("marshal SSE event %q: %w", ...), which the two callers in adapter_dispatch.go already propagate.

• config.decodeTypedExtensionConfig — data, _ := json.Marshal; _ = json.Unmarshal(data, typed) silently returned a partially/empty-populated typed struct on failure. Now logs via slog.Warn and falls back to the raw config map instead of a silently-broken struct.

• metrics.handleQuery / visual.ConfigForModel — both had if err == nil { _ = json.Unmarshal(data, &cfg) }, discarding the unmarshal error. Now log a warning and reset cfg = nil so the documented defaults are used deliberately rather than by accident.

• server.writeJSON — _ = json.NewEncoder(writer).Encode(payload) now logs on failure so a response that fails to serialize isn't invisible.

• app.runHTTPServer — the handler shutdown _ = closer.Close() now logs any close error via slog.Error.

Tests

Added unit tests:

• writeSSE propagates a marshal error (channel payload) and writes a well-formed frame for valid data.
• writeAuthJSON writes valid JSON and returns an error when the target parent path is a regular file.

go build, go vet, gofmt -l, and go test ./... all pass.

Link to Devin session: https://app.devin.ai/sessions/3573838eee674c6fb8f34a295153326a
Requested by: @ZhiYi-R

[devin-ai-integration]

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