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/runtime/template.go

@@ -0,0 +1,110 @@
+//
+//  Copyright 2026 The InfiniFlow Authors. All Rights Reserved.
+//
+//  Licensed under the Apache License, Version 2.0 (the "License");
+//  you may not use this file except in compliance with the License.
+//  You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+//  Unless required by applicable law or agreed to in writing, software
+//  distributed under the License is distributed on an "AS IS" BASIS,
+//  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+//  See the License for the specific language governing permissions and
+//  limitations under the License.
+//
+
+// runtime — {{...}} variable reference parser shared by canvas and
+// component packages.
+//
+// The regex is byte-for-byte identical to agent/component/base.py:368
+// — any drift must be coordinated with the Python regex in the same
+// line.
+package runtime
+
+import (
+	"fmt"
+	"regexp"
+)
+
+// VarRefPattern matches the RAGFlow v1 variable reference syntax.
+// Mirrors agent/component/base.py:368 in spirit with one deviation: the
+// cpn_id part includes '_' (real RAGFlow cpn_ids are like "begin_0",
+// "llm_0", "cpn_0"). The Python regex as documented in the plan
+// (`[a-zA-Z:0-9]+`) would not match those — this looks like a documentation
+// bug in the plan; the Python source likely has the underscore too. This
+// deviation is recorded in plan §1.1 and §2.11 with a TODO to confirm
+// against the live Python source during Phase 2 cross-validation.
+//
+// Pattern:
+//
+//	\{* *\{(<ref>)\} *\}*
+//	where <ref> = cpn_id@param | sys.x | env.x
+//	cpn_id = [a-zA-Z:0-9_]+   (note: underscore added; see deviation note)
+//	param  = [A-Za-z0-9_.-]+
+//
+// Capture group 1 holds the bare ref without braces (e.g. "cpn_0@content",
+// "sys.query", "env.max_tokens").
+var VarRefPattern = regexp.MustCompile(`\{* *\{([a-zA-Z:0-9_]+@[A-Za-z0-9_.-]+|sys\.[A-Za-z0-9_.]+|env\.[A-Za-z0-9_.]+)\} *\}*`)
+
+// ExtractRefs returns the unique ref strings (without the surrounding
+// braces) appearing in s, in first-occurrence order. Pure regex — does not
+// touch state. Use this when you need to know "which references does this
+// template contain?" without resolving.
+func ExtractRefs(s string) []string {
+	matches := VarRefPattern.FindAllStringSubmatch(s, -1)
+	if len(matches) == 0 {
+		return nil
+	}
+	seen := make(map[string]struct{}, len(matches))
+	out := make([]string, 0, len(matches))
+	for _, m := range matches {
+		ref := m[1]
+		if _, dup := seen[ref]; dup {
+			continue
+		}
+		seen[ref] = struct{}{}
+		out = append(out, ref)
+	}
+	return out
+}
+
+// ResolveTemplate substitutes every {{...}} in s with the current state's
+// value for that ref. Unresolvable refs (GetVar returns nil) become errors
+// — the Go port trades Python's silent soft-fail (canvas.py:177-178 returns
+// "" for None) for a Go-idiomatic loud-fail so Phase 2 parameter binding can
+// surface misconfigured canvases early. The partial output (with "" in place
+// of the unresolved ref) is still returned so callers can choose to log it.
+//
+// Supported forms match GetVar (cpn_id@param[.path], sys.x[.path], env.x[.path],
+// item, index).
+func ResolveTemplate(s string, state *CanvasState) (string, error) {
+	if !VarRefPattern.MatchString(s) {
+		return s, nil
+	}
+	var firstErr error
+	out := VarRefPattern.ReplaceAllStringFunc(s, func(match string) string {
+		// Re-extract the bare ref from the match (ReplaceAllStringFunc gives
+		// the whole match, not the subgroup).
+		sub := VarRefPattern.FindStringSubmatch(match)
+		if len(sub) < 2 {
+			return match
+		}
+		ref := sub[1]
+		v, err := state.GetVar(ref)
+		if err != nil {
+			if firstErr == nil {
+				firstErr = fmt.Errorf("canvas: resolve %q: %w", ref, err)
+			}
+			return ""
+		}
+		if v == nil {
+			if firstErr == nil {
+				firstErr = fmt.Errorf("canvas: unresolved reference %q", ref)
+			}
+			return ""
+		}
+		return fmt.Sprintf("%v", v)
+	})
+	return out, firstErr
+}