feat[Go]: port agent attachment download, chatbot + agentbot completion/info endpoints from Python (#16405)

## Summary

Ports five Python agent APIs to Go under the v1 Gin router:

- `GET  /api/v1/agents/attachments/<attachment_id>/download`
- `POST /api/v1/chatbots/<dialog_id>/completions`  (SSE)
- `GET  /api/v1/chatbots/<dialog_id>/info`
- `POST /api/v1/agentbots/<agent_id>/completions` (SSE)
- `GET  /api/v1/agentbots/<agent_id>/inputs`

Mirrors the existing Python wire shape (`{code, message,
data:{answer,reference,...}}` per Python `canvas_service.completion`) so
the iframe SDK and existing JS widgets keep working.

## Behavioural parity with Python

| # | Concern | How it's met |
|---|---------|--------------|
| R0 | Bot routes must not require regular user session | Routes mount
on `apiNoAuth` (router.go:198-202), with `BetaAuthMiddleware` only |
| R3 | Two SSE formats in Go drift | F2: `AgentChatCompletions` and
`AgentbotCompletion` share `service.WriteChatbotRunEvent` |
| R7 | `GetBySessionID` returns `(nil, nil)` on miss | Defensive
nil-check before `session.UserID != tenantID` |
| R8 | Begin component name vs ID | `FindBeginComponentID` resolves name
→ ID first, then `ExtractComponentInputForm(dsl, beginID)` |
| R9 | Defensive PromptConfig parsing | `stringFromMap` helper used for
`prologue` and `tavily_api_key` |
| R10 | `BetaAuthMiddleware` Bearer-prefix pre-filter | Removed —
`GetUserByToken` is called unconditionally, falls back to
`GetUserByBetaAPIToken` |
| F8 | Multi-turn chatbot history | `ChatbotCompletion` reads prior
turns from `session.Message`, appends user turn, calls LLM, persists new
pair via new `API4ConversationDAO.Update` |
| F9 | UUID gate stricter than plan | Removed — only `filepath.Base` +
CR/LF/quote header sanitization remains |
| H2 | Defence-in-depth IDOR | `AgentbotCompletion` calls `loadCanvas`
before delegating to `RunAgent` |
| M2 | SSE error leakage | `WriteChatbotFrame` emits generic `"an
internal error occurred"`; real error logged via `common.Error` |

## Verification

```bash
$ go vet ./...                                     # clean (only pre-existing issues)
$ go build ./...                                   # success
$ go test ./internal/handler/ ./internal/service/ ./internal/agent/dsl/ ./internal/common/ ./internal/dao/
ok  ragflow/internal/handler     0.617s
ok  ragflow/internal/service     1.729s
ok  ragflow/internal/agent/dsl   0.008s
ok  ragflow/internal/common      0.087s
ok  ragflow/internal/dao         0.083s
```

1199 tests pass across 5 packages.

## Known follow-ups (out of scope for this PR)

- **F1**: token-level streaming in `ChatbotCompletion` (currently emits
one frame per turn)
- **F3**: per-route `auth_types` attribute in Go (currently applied via
route group middleware)

---------

Co-authored-by: Claude <noreply@anthropic.com>

cmd/server_main.go

@@ -271,11 +271,21 @@ func startServer(config *server.Config) {
 	// is a 1-line if-not-nil pass-through — no separate "boot" mode
 	// required.
 	agentOpts := buildAgentRunOptions()
-	agentHandler := handler.NewAgentHandler(service.NewAgentServiceWithOptions(
+	agentService := service.NewAgentServiceWithOptions(
 		agentOpts.checkpointStore,
 		agentOpts.stateSerializer,
 		agentOpts.runTracker,
-	), fileService)
+	)
+	agentHandler := handler.NewAgentHandler(agentService, fileService)
+
+	// Public chatbot/agentbot endpoints (api/v1/chatbots/...,
+	// api/v1/agentbots/...) and the agent attachment download.
+	// BotService delegates the agentbot completion to agentService so
+	// both paths share the same canvas runner. Reuse the llmService
+	// already constructed above (line 222) — do NOT redeclare with
+	// `:=` since the variable is in scope.
+	botService := service.NewBotService(agentService, llmService)
+	botHandler := handler.NewBotHandler(botService)
 
 	// Wire the TTS synthesizer to the per-tenant model-provider
 	// dispatch. SynthesizeRequest is routed through
@@ -326,7 +336,7 @@ func startServer(config *server.Config) {
 	adminRuntimeHandler := handler.NewAdminRuntimeHandler(adminRuntimeSelector)
 
 	// Initialize router
-	r := router.NewRouter(authHandler, userHandler, tenantHandler, documentHandler, datasetsHandler, systemHandler, knowledgebaseHandler, chunkHandler, llmHandler, chatHandler, chatChannelHandler, langfuseHandler, chatSessionHandler, connectorHandler, searchHandler, fileHandler, memoryHandler, mcpHandler, skillSearchHandler, providerHandler, agentHandler, searchBotHandler, difyRetrievalHandler, pluginHandler, modelHandler, fileCommitHandler, adminRuntimeHandler, openaiChatHandler)
+	r := router.NewRouter(authHandler, userHandler, tenantHandler, documentHandler, datasetsHandler, systemHandler, knowledgebaseHandler, chunkHandler, llmHandler, chatHandler, chatChannelHandler, langfuseHandler, chatSessionHandler, connectorHandler, searchHandler, fileHandler, memoryHandler, mcpHandler, skillSearchHandler, providerHandler, agentHandler, searchBotHandler, difyRetrievalHandler, pluginHandler, modelHandler, fileCommitHandler, adminRuntimeHandler, openaiChatHandler, botHandler)
 
 	// Create Gin engine
 	ginEngine := gin.New()