refactor(server): drop stale prompt text, duplicated routes and dead options

- lib/system-prompts.ts: delete the extended prompt kept only to reach
  the 4096-token cache threshold of two old Claude models; describe the
  current canvas-first UI in the App Features section
- app/api/chat/route.ts: remove the server-side example cache (the
  browser answers examples itself), the ENABLE_HISTORY_XML_REPLACE
  option and its helper, the unread messageMetadata fields; the
  DEBUG_LLM_PAYLOAD blocks become one describeMessages summary
- app/api/log-feedback and log-save share scoreLatestTrace in
  lib/langfuse.ts
- app/api/admin/test-model and validate-model call a shared
  testModelConnection() instead of one route faking a request to the
  other and passing trust through the body
- lib/dynamo-quota-manager.ts reads the failing item from the
  conditional update instead of a second GetItem
- lib/ai-providers.ts: geminiThinkingOptions helper for the google and
  vertexai cases, drop the unused fourth resolveBaseURL parameter
- edge-functions: always stream, shrink messageSchema, drop model
  aliases nothing sends
- delete app/api/chat/xml_guide.md, parseOutputTokenLimit, unused type
  exports and export keywords on file-internal symbols
- env.example: drop variables nothing reads, document the quota,
  validation and analytics variables, mention the apiKeyEnv array form
  (also in docs/*/ai-providers.md)

tests/unit/log-save-route.test.ts

@@ -2,9 +2,11 @@
 import { describe, expect, it, vi } from "vitest"
 import { POST as logSave } from "@/app/api/log-save/route"
 
-// A Langfuse client with a trace to attach the save score to
+// A Langfuse client with a trace to attach the save score to; the real
+// scoreLatestTrace runs against it
 const batch = vi.fn(async () => ({}))
-vi.mock("@/lib/langfuse", () => ({
+vi.mock("@/lib/langfuse", async (importOriginal) => ({
+    ...(await importOriginal<typeof import("@/lib/langfuse")>()),
     getLangfuseClient: () => ({
         api: {
             trace: { list: async () => ({ data: [{ id: "trace-1" }] }) },