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/persona_chat.rs

@@ -37,8 +37,8 @@ use uuid::Uuid;
 use crate::ai::backend::{BackendKind, SamplingOverrides};
 use crate::ai::handlers::ReplayQuery;
 use crate::ai::insight_chat::{
-    BYTES_PER_TOKEN, ChatStreamEvent, DEFAULT_MAX_ITERATIONS, ForkInfo, RESPONSE_HEADROOM_TOKENS,
-    apply_context_budget, env_default_num_ctx, env_max_iterations, render_tree_path,
+    ChatStreamEvent, ContextFitter, DEFAULT_MAX_ITERATIONS, ForkInfo, env_max_iterations,
+    render_tree_path,
 };
 use crate::ai::insight_generator::InsightGenerator;
 use crate::ai::llm_client::{ChatHistoryStore, ChatMessage, StoredChatNode};
@@ -842,10 +842,8 @@ impl PersonaChatSession {
         // Trim to the model's context window before the turn. Without this a
         // rolling persona transcript grows unbounded until the backend
         // silently drops the front of it mid-conversation.
-        let budget_tokens = (req.num_ctx.unwrap_or_else(env_default_num_ctx) as usize)
-            .saturating_sub(RESPONSE_HEADROOM_TOKENS);
-        let budget_bytes = budget_tokens.saturating_mul(BYTES_PER_TOKEN);
-        let truncated = apply_context_budget(&mut messages, budget_bytes);
+        let mut fitter = ContextFitter::for_client(backend.chat(), req.num_ctx).await;
+        let truncated = fitter.trim(&mut messages);
         if truncated {
             let _ = entry.push_event(ChatStreamEvent::Truncated).await;
         }
@@ -880,6 +878,7 @@ impl PersonaChatSession {
             &persona.persona_id,
             max_iterations,
             &entry,
+            &mut fitter,
         )
         .await
         .map_err(|e| anyhow!("persona chat loop failed: {e:?}"))?;