### What problem does this PR solve?
Refs #15743
Some Go API handlers return raw `err.Error()` strings in
`CodeServerError` responses. Those errors can include internal backend
details such as database, storage, search engine, or host information.
This PR adds a small shared `jsonInternalError` helper for handler-level
internal failures. The helper logs the raw error server-side with
request method/path context, then returns the existing generic
`common.CodeServerError.Message()` to API clients.
This first slice migrates the existing `jsonError(c,
common.CodeServerError, err.Error())` production call sites in agent,
dataset graph, file, and system handlers. It intentionally does not
close the full issue because direct `c.JSON` error responses in other
handlers remain for follow-up PRs.
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
- [ ] New Feature (non-breaking change which adds functionality)
- [ ] Documentation Update
- [ ] Refactoring
- [ ] Performance Improvement
- [ ] Other (please describe):
### Tests
- `/root/go/bin/go test ./internal/handler -count=1`
---------
Co-authored-by: Yingfeng <yingfeng.zhang@gmail.com>
