fix(chat): budget chat history against the model's served context window

Long insight/persona chats on llama-swap failed with "exceeds the
available context size" even after raising Context in the app:

- llama-server fixes its window at launch (-c) and ignores num_ctx, but
  the history budget trusted the requested num_ctx, so raising it kept
  MORE history than the server could hold. LlmClient::context_window()
  now reports the served window (llama-swap: /upstream/<model>/props,
  cached 10 min) and the budget uses min(requested, served).
- apply_context_budget could only drop tool-call pairs, so plain Q&A
  chats never shrank. It now also drops whole earlier exchanges (keeping
  user/assistant alternation) and, last, the current turn's tool results,
  never the in-progress question.
- The budget only ran once, before the tool loop. ContextFitter now
  fits every LLM call (on a copy; the stored transcript is untouched)
  and, on a context-overflow error, shrinks the budget from the server's
  reported token counts and retries once.

GET /insights/models reports each llama-swap model's context_window when
it's known without loading the model (cached, or currently running), for
the app's Context picker.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/ai/handlers.rs

@@ -995,6 +995,8 @@ pub async fn generate_agentic_insight_handler(
 /// configured `LLAMA_SWAP_VISION_MODEL` slot id, and `has_tool_calling` is
 /// reported as true for every slot (llama-server is launched with `--jinja`
 /// by convention — a misconfigured slot surfaces as a chat-call error).
+/// `context_window` is each slot's served `-c` window when known without
+/// loading the model (cached from a recent chat, or the model is running).
 #[get("/insights/models")]
 pub async fn get_available_models_handler(
     _claims: Claims,
@@ -1005,6 +1007,10 @@ pub async fn get_available_models_handler(
     if crate::ai::local_backend_is_llamacpp()
         && let Some(lc) = app_state.llamacpp.as_ref()
     {
+        // Served windows for the context picker; never loads a model.
+        let windows = lc
+            .known_context_windows(&app_state.llamacpp_allowed_models)
+            .await;
         let models: Vec<ModelCapabilities> = app_state
             .llamacpp_allowed_models
             .iter()
@@ -1012,6 +1018,7 @@ pub async fn get_available_models_handler(
                 name: name.clone(),
                 has_vision: true,
                 has_tool_calling: true,
+                context_window: windows.get(name).copied(),
             })
             .collect();
         let primary = ServerModels {