feat(agent): Go port — canvas engine, 22 components, DSL v2, 13 endpoints (#15952)

Ports the agent canvas subsystem from Python to Go.

## What's included

### Canvas Engine (Phase 0/1)
- State engine, scheduler, variable resolver, Redis checkpoint store,
cancel protocol
- **209 tests** across canvas / component / io packages

### 22 Components (P0–P4)
| Tier | Components |
|---|---|
| P0 T1+T2+T3 | LLM, Agent, ExitLoop, Switch, Categorize, Begin,
Message, Invoke |
| P1 T3 | VariableAggregator, VariableAssigner, StringTransform,
ListOperations, DataOperations |
| P2 T3 | Iteration, IterationItem, Loop, LoopItem |
| P3 T3 | UserFillUp, Fillup |
| P4 T5 | Browser, ExcelProcessor, DocsGenerator |

### DSL v2 Schema (Phase 2.5)
- Typed v2 in-memory model with v1-to-v2 auto-detect converter
- v1 legacy field stripping per plan §2.11.7

### HTTP Endpoints & Bug Fixes (Plans PR1–PR3)
- **DELETE SQL bug fix**: gorm v2 `Where("id = ?", id).Delete(...)`
pattern
- **CreateAgent validation**: title/DSL required, duplicate check, 103
envelope
- **13 new endpoints**: templates, prompts, tags, sessions CRUD,
chat/completions (SSE + non-stream stubs), rerun, test_db_connection,
logs, webhook/logs
- **756 Go unit tests** (745 → 756, +18)
- **17 → 0 Python integration test failures** (test_agents.py +
test_session_management/)

### Tools
21 eino tools: HTTPHelper, search tools, financial/data tools, mandatory
stubs

### Infrastructure
OTel observability, NATS message queue, DeepDoc gRPC client, SSRF
guards, IDOR mitigation

internal/handler/providers.go

@@ -1585,3 +1585,40 @@ func (h *ProviderHandler) ParseFile(c *gin.Context) {
 		"message": "success",
 	})
 }
+
+// ListTenantAddedModels is the response handler for GET /api/v1/models.
+// It is the Go port of Python's
+// api/apps/restful_apis/models_api.py:get_added_models and feeds
+// web/src/hooks/use-llm-request.tsx → useFetchAllAddedModels. The data
+// shape is the array form (one row per (provider × instance × llm) with
+// model_type: string[]), matching the IAddedModel interface in
+// web/src/interfaces/database/llm.ts:64-71.
+//
+// The previous contract routed this path to TenantHandler.GetModels →
+// TenantService.ListTenantDefaultModels, which only enumerates the 6-7
+// default tenant fields and returned `[]` for any tenant without
+// defaults, breaking the front-end's "View Models" list. The Go port
+// has no writers for tenant_model, so this endpoint must be driven by
+// the factory catalog cross-referenced with the tenant's instance list —
+// see service.ModelProviderService.ListTenantAddedModels.
+func (h *ProviderHandler) ListTenantAddedModels(c *gin.Context) {
+	user, errorCode, errorMessage := GetUser(c)
+	if errorCode != common.CodeSuccess {
+		jsonError(c, errorCode, errorMessage)
+		return
+	}
+
+	modelType := c.Query("type")
+
+	addedModels, code, err := h.modelProviderService.ListTenantAddedModels(user.ID, modelType)
+	if err != nil {
+		jsonError(c, code, err.Error())
+		return
+	}
+
+	c.JSON(http.StatusOK, gin.H{
+		"code":    0,
+		"data":    addedModels,
+		"message": "success",
+	})
+}