refactor: add provider_override for pipeline dependency injection

Adds an optional LlmProvider override to run_generation and
run_generation_inner, allowing tests to inject a mock provider without
touching real credentials or the provider-resolution path. Makes
run_generation_inner pub so integration tests can call it directly.
Production callers pass None and behaviour is unchanged.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/handlers/generation.rs

@@ -76,7 +76,7 @@ pub async fn trigger_generate(
 
     let join_handle = tokio::spawn(async move {
         let timeout_duration = std::time::Duration::from_secs(900);
-        match tokio::time::timeout(timeout_duration, synthesis::run_generation(job_id, state_clone.clone(), user_id, tx.clone())).await {
+        match tokio::time::timeout(timeout_duration, synthesis::run_generation(job_id, state_clone.clone(), user_id, tx.clone(), None)).await {
             Ok(()) => {}
             Err(_) => {
                 tracing::error!(job_id = %job_id, user_id = %user_id, "Generation timed out after 15 minutes");

backend/src/services/synthesis.rs

@@ -194,8 +194,9 @@ pub async fn run_generation(
     state: AppState,
     user_id: Uuid,
     tx: Arc<watch::Sender<ProgressEvent>>,
+    provider_override: Option<Arc<dyn crate::services::llm::LlmProvider>>,
 ) {
-    let result = run_generation_inner(job_id, &state, user_id, &tx).await;
+    let result = run_generation_inner(job_id, &state, user_id, &tx, provider_override).await;
 
     match result {
         Ok(synthesis_id) => {
@@ -224,11 +225,12 @@ pub async fn run_generation(
 }
 
 /// Inner implementation of the generation pipeline, returning a Result.
-async fn run_generation_inner(
+pub async fn run_generation_inner(
     job_id: Uuid,
     state: &AppState,
     user_id: Uuid,
     tx: &watch::Sender<ProgressEvent>,
+    provider_override: Option<Arc<dyn crate::services::llm::LlmProvider>>,
 ) -> Result<Uuid, AppError> {
     // Batch buffer for article history traces (flushed at logical boundaries)
     let mut pending_traces: Vec<db::article_history::ArticleHistoryEntry> = Vec::new();
@@ -254,10 +256,22 @@ async fn run_generation_inner(
     let sources = db::sources::list_for_user(&state.pool, user_id).await?;
 
     emit_progress(tx, "provider", "Configuration du fournisseur IA...", 12);
-    let (provider_name, api_key) = resolve_provider_and_key(state, user_id, &settings).await?;
+    let (provider_name, provider) = if let Some(mock_provider) = provider_override {
-    let provider = create_provider(&provider_name, api_key)?;
+        ("mock".to_string(), mock_provider)
+    } else {
+        let (pname, api_key) = resolve_provider_and_key(state, user_id, &settings).await?;
+        let p = create_provider(&pname, api_key)?;
+        (pname, p)
+    };
+    let (model_research, model_websearch) = if provider_name == "mock" {
+        let research = if settings.ai_model.is_empty() { "mock-model".to_string() } else { settings.ai_model.clone() };
+        let websearch = if settings.ai_model_websearch.is_empty() { "mock-model".to_string() } else { settings.ai_model_websearch.clone() };
+        (research, websearch)
+    } else {
         let model_research = if !settings.ai_model.is_empty() { settings.ai_model.clone() } else { resolve_model(state, &provider_name).await? };
         let model_websearch = if !settings.ai_model_websearch.is_empty() { settings.ai_model_websearch.clone() } else { model_research.clone() };
+        (model_research, model_websearch)
+    };
     let user_rate_limiter = get_user_rate_limiter(state, &settings, user_id);
 
     // Tracking structures