fix(server): keep OpenCode timeouts from crashing the error formatter - #12469

Eligibility
This is a small, self-contained server bug fix that prevents OpenCode error formatting from crashing on missing or non-string messages, with focused regression coverage and no default, schema, security, or static-analysis changes. The supplied High-severity finding’s requested single getter read is already implemented and tested in the current head, though the unresolved finding remains independently blocking under the repository threshold.

Notes:
No code owners found in target merge base c616e16