### What problem does this PR solve?
Add two API in go
```
/api/v1/agents/test_db_connection POST
/api/v1/agents/<agent_id>/sessions DELETE
```
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
---------
Co-authored-by: Yingfeng <yingfeng.zhang@gmail.com>
### What problem does this PR solve?
As Title
Codes were tested by Postman
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
## Summary
Port the canvas-template catalogue endpoint to the Go API server. Listed
in the Go-API port checklist of #15240.
Mirrors `list_agent_template` in `api/apps/restful_apis/agent_api.py`:
returns every row from the `canvas_template` table so that the UI can
render the template gallery on the New-Agent screen.
## What
- `internal/dao/canvas_template.go` — new `CanvasTemplateDAO.GetAll()`
ordered by `create_time desc` (newest templates first).
- `internal/service/agent.go` — wire the new DAO into `AgentService` and
expose `ListTemplates() ([]*entity.CanvasTemplate, error)`.
- `internal/handler/agent.go` — new `AgentHandler.ListTemplates` HTTP
handler (auth-gated, mirrors Python `@login_required`).
- `internal/router/router.go` — `agents.GET("/templates",
r.agentHandler.ListTemplates)` registered alongside the existing `GET
/agents`.
- `internal/handler/agent_test.go` — three new tests covering: success
path, empty-list → JSON array (not `null`), and the auth gate.
## Notes
- `CanvasTemplate` entity, GORM tags, and DB migration already exist in
`internal/entity/canvas.go` and `internal/dao/database.go` — no schema
change required.
- The handler coerces a `nil` slice to `[]*entity.CanvasTemplate{}` so
the JSON payload is always an array (the frontend does `data.map(...)`
on it).
## Test plan
- [x] `go vet ./internal/handler ./internal/service ./internal/dao
./internal/router` clean
- [x] Three unit tests added; existing `TestListAgents_Success`
untouched
- [ ] CI runs `go test ./internal/handler` with cgo binding linked
## Related
- Tracker: #15240
## Summary
Closes: #15328
- Implements `GET /api/v1/agents` — the agent/canvas listing endpoint
needed to complete the Home dashboard tile in `web/src/pages/home/`.
- Mirrors Python `api/apps/restful_apis/agent_api.py::list_agents`
exactly: tenant-join auth, optional `owner_ids` guard, keyword filter,
pagination, ordering, and `canvas_category` filter (default:
`agent_canvas`).
- **Scope:** read-only list only. Full agent CRUD and canvas runtime are
explicitly out of scope (separate slice of #15240).
