Port 14 upstream agent security / correctness fixes to Go canvas (#16455)

Mirrors 14 merged upstream PRs into the Go agent port.

PRs ported:
  - #15609 ExeSQL SSRF guard + DNS pin
  - #15436 HTTP timeout on external API tools
  - #16363 be_output restore + DeepL error path
  - #15644 switch no longer matches empty condition
  - #15374 session_id bind to path agent_id (DAO idor guard)
  - #16169 sandbox artifact ownership gate
  - #15457 tenant ownership on agentbots
  - #15145 rerun agent document access check
- #15446 thinking switch (component portion; provider policy lives in
internal/llm)
  - #15426 Invoke URL/proxy SSRF + DNS pin + no-redirects
  - #15238 agentbot thinking-logs beta endpoint
  - #14589 UserFillUp SSE event propagation
  - #14890 anonymous webhook opt-in
- #15068 PipelineChunker new component (text/file_ref/parser_id
dispatch; file-format extraction is a follow-up)

40 files, +2355 / -58 lines. 33 new tests, all targeted package suites
pass (1721 + 4 skipped); 1 pre-existing flaky test unrelated.

cmd/server_main.go

@@ -277,7 +277,8 @@ func startServer(config *server.Config) {
 		agentOpts.stateSerializer,
 		agentOpts.runTracker,
 	)
-	agentHandler := handler.NewAgentHandler(agentService, fileService)
+	agentHandler := handler.NewAgentHandler(agentService, fileService).
+		WithDocumentService(documentService)
 
 	// Public chatbot/agentbot endpoints (api/v1/chatbots/...,
 	// api/v1/agentbots/...) and the agent attachment download.