fix(chat): use the AI SDK's own mechanisms for quota, retries, stops and notices

Server
- Record quota tokens in onStepFinish and await the write, so a step that
  fails or is stopped later still counts; drop the onFinish/onAbort copies
- validate-diagram and the connection test end their model call with the
  request (req.signal) and use the SDK's timeout option; validate-diagram
  reports to Langfuse (functionId "validate-diagram")
- The connection test no longer counts an invalid tool call as support
- Validate incoming messages with safeValidateUIMessages (400 otherwise),
  drop client system messages explicitly, prune empty model messages with
  pruneMessages
- Chat tools move to lib/chat-tools.ts with an output schema for the
  client-side tools

Client
- Every automatic retry sends the canvas as it is then
  (prepareSendMessagesRequest): a drawing is committed before its
  screenshot check can fail, so the model must go on from it
- A notice when the output budget ran out before any tool call
  (finishReason "length")
- Error notices are assistant messages with a data-notice part instead of
  role "system" messages (lib/chat-message.ts: ChatUIMessage with metadata,
  data part and InferUITools types)
- regenerate, edit and retry use sendMessage({ messageId }); isResending
  and flushSync go
- isToolUIPart/getToolName instead of slicing "tool-"; ToolPartLike derives
  from the SDK's part types; addToolOutput typed with ChatAddToolOutputFunction

Tests: step failure after a finished step, abort of the screenshot check,
invalid tool call in the connection test, malformed messages.

lib/langfuse.ts

@@ -1,6 +1,7 @@
 import { LangfuseClient } from "@langfuse/client"
 import { observe, updateActiveTrace } from "@langfuse/tracing"
 import * as api from "@opentelemetry/api"
+import type { TelemetrySettings } from "ai"
 import { randomUUID } from "crypto"
 
 // Singleton LangfuseClient instance for direct API calls
@@ -115,20 +116,23 @@ export function endTrace() {
     }
 }
 
-// Get telemetry config for streamText
+// Telemetry settings for streamText and generateText
 export function getTelemetryConfig(params: {
     sessionId?: string
     userId?: string
-}) {
+    /** Names the trace of a model call outside the chat's observe() span */
+    functionId?: string
+}): TelemetrySettings | undefined {
     if (!isLangfuseEnabled()) return undefined
 
     return {
         isEnabled: true,
+        functionId: params.functionId,
         recordInputs: true,
         recordOutputs: true,
         metadata: {
-            sessionId: params.sessionId,
-            userId: params.userId,
+            ...(params.sessionId && { sessionId: params.sessionId }),
+            ...(params.userId && { userId: params.userId }),
         },
     }
 }