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/agent/canvas/variable.go

@@ -0,0 +1,24 @@
+// Package canvas — variable reference helpers (re-exports).
+//
+// The canonical VarRefPattern / ExtractRefs / ResolveTemplate
+// implementations live in internal/agent/runtime/template.go so
+// components can depend on them without importing canvas. This file
+// re-exports the symbols for callers that already use canvas.X.
+package canvas
+
+import (
+	"ragflow/internal/agent/runtime"
+)
+
+// VarRefPattern aliases runtime.VarRefPattern.
+var VarRefPattern = runtime.VarRefPattern
+
+// ExtractRefs re-exports runtime.ExtractRefs.
+func ExtractRefs(s string) []string {
+	return runtime.ExtractRefs(s)
+}
+
+// ResolveTemplate re-exports runtime.ResolveTemplate.
+func ResolveTemplate(s string, state *CanvasState) (string, error) {
+	return runtime.ResolveTemplate(s, state)
+}