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

pyproject.toml

@@ -5,7 +5,7 @@ description = "[RAGFlow](https://ragflow.io/) is an open-source RAG (Retrieval-A
 authors = [{ name = "Zhichang Yu", email = "yuzhichang@gmail.com" }]
 license-files = ["LICENSE"]
 readme = "README.md"
-requires-python = ">=3.13,<3.15"
+requires-python = ">=3.13,<3.14"
 dependencies = [
     # discord-py==2.3.2 unconditionally imports audioop in discord/player.py at module-
     # load time. audioop was removed from the CPython stdlib in Python 3.13 (PEP 594),
@@ -155,7 +155,12 @@ dependencies = [
     #    "xxhash>=3.5.0,<4.0.0",
     #    "trio>=0.17.0,<0.29.0",
     #    "click>=8.1.8",
-    "litellm==1.88.0",
+    # 1.88.0 ships a broken `litellm.integrations.custom_logger`
+    # (imports `AgenticLoopPlan` from a sibling types module where the
+    # symbol is missing — see PR #15952 CI run 27414495913), and the
+    # 1.82.6/1.82.7/1.82.8 minors have a broken `litellm.caching.caching`
+    # import chain. Pin to a single known-good 1.82.x release.
+    "litellm==1.82.5",
     #    "pip>=25.2",
     #    "imageio-ffmpeg>=0.6.0",
     #    "cryptography==46.0.3",