insight-chat: extract bootstrap resolution helpers + unit-test them

resolve_bootstrap_system_prompt and resolve_bootstrap_backend run on
every bootstrap turn — they pick the persisted system prompt and the
chosen backend label. They were inline conditionals before; pulling
them out makes the rules testable without spinning up the full
streaming stack.

9 new tests cover:
- system prompt fallback to BOOTSTRAP_DEFAULT_SYSTEM_PROMPT for None,
  empty string, whitespace-only
- supplied non-empty prompts pass through verbatim, with interior
  newlines / spacing preserved (Apollo personas use multi-line tool
  listings)
- backend defaults to "local" for None / empty
- "local" / "hybrid" accepted case-insensitively with edge-trim
- unknown labels return a descriptive error

Total insight_chat tests: 24 (up from 15). No behaviour change.

src/ai/insight_chat.rs

@@ -915,18 +915,7 @@ impl InsightChatService {
         normalized: String,
         tx: tokio::sync::mpsc::Sender<ChatStreamEvent>,
     ) -> Result<()> {
-        let effective_backend = req
-            .backend
-            .as_deref()
-            .map(|s| s.trim().to_lowercase())
-            .filter(|s| !s.is_empty())
-            .unwrap_or_else(|| "local".to_string());
-        if !matches!(effective_backend.as_str(), "local" | "hybrid") {
-            bail!(
-                "unknown backend '{}'; expected 'local' or 'hybrid'",
-                effective_backend
-            );
-        }
+        let effective_backend = resolve_bootstrap_backend(req.backend.as_deref())?;
         let is_hybrid = effective_backend == "hybrid";
 
         let max_iterations = req
@@ -976,13 +965,7 @@ impl InsightChatService {
         let tools = InsightGenerator::build_tool_definitions(gate_opts);
 
         // Build initial messages: persona system + user-turn-with-image.
-        let system_content = req
-            .system_prompt
-            .as_deref()
-            .map(str::trim)
-            .filter(|s| !s.is_empty())
-            .unwrap_or(BOOTSTRAP_DEFAULT_SYSTEM_PROMPT)
-            .to_string();
+        let system_content = resolve_bootstrap_system_prompt(req.system_prompt.as_deref());
         let system_msg = ChatMessage::system(system_content);
         let mut user_msg = ChatMessage::user(user_content);
         if !is_hybrid && let Some(ref img) = image_base64 {
@@ -1297,6 +1280,34 @@ const BOOTSTRAP_DEFAULT_SYSTEM_PROMPT: &str = "You are a helpful AI assistant an
      Use the available tools to gather context and answer their questions \
      in a conversational tone.";
 
+/// Pick the system prompt for bootstrap. Trimmed-non-empty supplied wins;
+/// otherwise fall back to [`BOOTSTRAP_DEFAULT_SYSTEM_PROMPT`]. Returns an
+/// owned `String` because the bootstrap caller persists it on the new
+/// insight row.
+fn resolve_bootstrap_system_prompt(supplied: Option<&str>) -> String {
+    supplied
+        .map(str::trim)
+        .filter(|s| !s.is_empty())
+        .map(str::to_string)
+        .unwrap_or_else(|| BOOTSTRAP_DEFAULT_SYSTEM_PROMPT.to_string())
+}
+
+/// Pick the backend label for bootstrap. Bootstrap has no stored insight
+/// to defer to (that's continuation's behaviour), so the default is
+/// `"local"`. Returns an error if the supplied label is non-empty but
+/// not one of the recognised values — same surface as continuation's
+/// validation.
+fn resolve_bootstrap_backend(supplied: Option<&str>) -> Result<String> {
+    let lower = supplied
+        .map(|s| s.trim().to_lowercase())
+        .filter(|s| !s.is_empty())
+        .unwrap_or_else(|| "local".to_string());
+    if !matches!(lower.as_str(), "local" | "hybrid") {
+        bail!("unknown backend '{}'; expected 'local' or 'hybrid'", lower);
+    }
+    Ok(lower)
+}
+
 /// Outcome of one streaming agentic loop pass. Shared between bootstrap
 /// and continuation.
 struct AgenticLoopOutcome {
@@ -1840,4 +1851,73 @@ mod tests {
         assert_eq!(msgs.len(), 1);
         assert_eq!(msgs[0].role, "user");
     }
+
+    // ── Bootstrap prompt / backend resolution ─────────────────────────
+    // Resolution helpers run on every bootstrap turn — they pick the
+    // persisted system prompt and the chosen backend label. Bugs here
+    // would silently swap the persona or miscategorise a backend.
+
+    #[test]
+    fn bootstrap_system_prompt_falls_back_to_default_for_none() {
+        let out = resolve_bootstrap_system_prompt(None);
+        assert_eq!(out, BOOTSTRAP_DEFAULT_SYSTEM_PROMPT);
+    }
+
+    #[test]
+    fn bootstrap_system_prompt_falls_back_to_default_for_empty_string() {
+        // Apollo currently sends `''` when no persona is selected.
+        let out = resolve_bootstrap_system_prompt(Some(""));
+        assert_eq!(out, BOOTSTRAP_DEFAULT_SYSTEM_PROMPT);
+    }
+
+    #[test]
+    fn bootstrap_system_prompt_falls_back_to_default_for_whitespace() {
+        let out = resolve_bootstrap_system_prompt(Some("   \n\t  "));
+        assert_eq!(out, BOOTSTRAP_DEFAULT_SYSTEM_PROMPT);
+    }
+
+    #[test]
+    fn bootstrap_system_prompt_uses_supplied_when_non_empty() {
+        let out = resolve_bootstrap_system_prompt(Some("you are a journal"));
+        assert_eq!(out, "you are a journal");
+    }
+
+    #[test]
+    fn bootstrap_system_prompt_does_not_strip_inner_whitespace() {
+        // Trim only happens at the edges — interior newlines and spacing
+        // (which Apollo's persona uses for tool listings) must survive.
+        let prompt = "line one\nline two\n  bullet";
+        let out = resolve_bootstrap_system_prompt(Some(prompt));
+        assert_eq!(out, prompt);
+    }
+
+    #[test]
+    fn bootstrap_backend_defaults_to_local_when_none() {
+        let out = resolve_bootstrap_backend(None).unwrap();
+        assert_eq!(out, "local");
+    }
+
+    #[test]
+    fn bootstrap_backend_defaults_to_local_when_empty() {
+        let out = resolve_bootstrap_backend(Some("")).unwrap();
+        assert_eq!(out, "local");
+    }
+
+    #[test]
+    fn bootstrap_backend_accepts_local_and_hybrid_case_insensitively() {
+        assert_eq!(resolve_bootstrap_backend(Some("LOCAL")).unwrap(), "local");
+        assert_eq!(resolve_bootstrap_backend(Some("Hybrid")).unwrap(), "hybrid");
+        assert_eq!(
+            resolve_bootstrap_backend(Some("  local  ")).unwrap(),
+            "local"
+        );
+    }
+
+    #[test]
+    fn bootstrap_backend_rejects_unknown_label() {
+        let err = resolve_bootstrap_backend(Some("openrouter")).unwrap_err();
+        let msg = format!("{}", err);
+        assert!(msg.contains("unknown backend"));
+        assert!(msg.contains("openrouter"));
+    }
 }