fix(rag): use a stable hash for document IDs so dedup survives restarts (#1098)

add_document() and add_documents_batch() derive the persistent ChromaDB
document id from Python's built-in hash():

    doc_id = f"doc_{hash(text) % 10**16}"

str hashing is randomized per process (PYTHONHASHSEED is on by default), so
the same document text gets a different doc_id on every restart. The dedup
check right after — self._collection.get(ids=[doc_id]) — therefore misses
on restart, and identical documents are re-embedded and re-added as
duplicates each time the app restarts, bloating the vector store and
skewing retrieval.

Derive the id from a stable hashlib.sha256 of the text via a shared
_generate_doc_id() helper, used by both add paths so they agree.

tests/test_rag_vector_id_stability.py runs _generate_doc_id in subprocesses
under PYTHONHASHSEED=0/1/random and asserts the id is identical across all
of them (and differs for different text). Fails before this change.

tests/test_rag_vector_id_stability.py

@@ -0,0 +1,28 @@
+import os
+import subprocess
+import pytest
+
+def test_rag_id_stability_across_processes():
+    # Run helper in subprocesses with different PYTHONHASHSEED values to ensure cross-process stability
+    cmd = ["./venv/bin/python", "-c", "from src.rag_vector import _generate_doc_id; print(_generate_doc_id('test_text_hash'))"]
+    
+    env0 = os.environ.copy()
+    env0["PYTHONHASHSEED"] = "0"
+    id0 = subprocess.check_output(cmd, env=env0).decode().strip()
+    
+    env1 = os.environ.copy()
+    env1["PYTHONHASHSEED"] = "1"
+    id1 = subprocess.check_output(cmd, env=env1).decode().strip()
+    
+    env_rand = os.environ.copy()
+    env_rand["PYTHONHASHSEED"] = "random"
+    id_rand = subprocess.check_output(cmd, env=env_rand).decode().strip()
+    
+    # Assert they are all equal (deterministic across seeds and processes)
+    assert id0 == id1
+    assert id0 == id_rand
+    
+    # Assert different inputs produce different IDs
+    cmd_diff = ["./venv/bin/python", "-c", "from src.rag_vector import _generate_doc_id; print(_generate_doc_id('different_text_hash'))"]
+    id_diff = subprocess.check_output(cmd_diff, env=env0).decode().strip()
+    assert id0 != id_diff