fix: handle string body in test executor to avoid double JSON encoding

[mikolajbadyl]

The agent sends request bodies as JSON strings (per the ExecuteTestGroup tool schema: "type": ["string", "null"]). The executor was calling json.Marshal on that string, which double-encoded it — the server received "{\name\:\"Alice\"}" instead of {"name":"Alice"}, causing all POST/PUT/PATCH requests to fail with 400.

Fix: if body is already a string, pass it directly to the request reader instead of marshaling it again.

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Files Reviewed (1 file)

• internal/core/tester/executor.go - No issues

Analysis:
The PR correctly adds string body handling by using a type switch:

• When body is a non-empty string, it uses strings.NewReader(b) directly
• For other types, it marshals to JSON as before

The Content-Type: application/json header is unchanged in this PR (appears as context line in diff), so any concerns about header/body mismatch are pre-existing and outside the scope of this change.