fix(rag): surface embedding provider init failure as 'error' status

The bug: when the local embedding provider fails to initialize
(e.g. sentence-transformers not installed), RAGEngine.__init__
leaves self.embedding_provider = None (initialized at line 93
but never overwritten by the failing LocalEmbeddingProvider ctor).
The constructor returns. _sync_rag_engine's else branch then
sets status to 'ready' - a lie. The RAG panel shows 'ready'.
The user triggers a retrieval. The engine either has a broken
embedding provider (None) or the retrieval fails silently.
The RAG context never appears in the AI's history.

The fix: in _sync_rag_engine's _task, after RAGEngine(...)
returns, check if engine.embedding_provider is None. If so,
set status to 'error: RAG embedding provider failed to initialize'
and return early. This prevents:
  - The engine from being assigned to self.rag_engine
  - The rebuild being triggered
  - The status being set to 'ready' / 'indexing'

Note: this does NOT make the RAG test pass. The test requires
the sentence-transformers package which isn't installed in this
env. The fix makes the failure reliable (not flaky) and surfaces
the right error message.

TDD: 3 tests added in tests/test_rag_engine_ready_status_bug.py:
- RAGEngine ctor raises ImportError on missing sentence-transformers
- _sync_rag_engine sets status to 'error' (not 'ready') on init failure
- RAGEngine ctor leaves embedding_provider=None when init fails

All 3 pass. The RAG batch test now fails reliably at line 46
with the clear error message.

src/app_controller.py

@@ -1466,6 +1466,14 @@ class AppController:
    try:
     from src import rag_engine
     engine = rag_engine.RAGEngine(self.rag_config, self.active_project_root)
+    # If the engine's embedding provider failed to initialize
+    # (e.g. local embedding but sentence-transformers not installed),
+    # the engine is in a broken state even though __init__ returned.
+    # Surface this as an error instead of reporting 'ready' (which
+    # would let the user trigger RAG queries that silently fail).
+    if engine.embedding_provider is None:
+     self._set_rag_status("error: RAG embedding provider failed to initialize (e.g. missing dependencies)")
+     return
     with self._rag_engine_lock:
      self.rag_engine = engine
     # If the engine is empty and we have files, trigger indexing