Default retrieval to 6 chunks instead of 2

chunks is a total per request, split across the attached sources, so an
agent with two sources and the default of 2 got a single chunk from each,
and its answers changed with whichever chunk won. 6 gives three per
source for about 4-5k more input tokens per retrieval turn.

Every literal default moves from 2 to 6: the request default, the
retrievers, the internal search tool, workflow agent nodes, scheduled and
headless runs, agent create/update/import, the source retrieval config and
the frontend forms. Existing agents and sources keep what they store; a
source saved with chunks=2 now counts as configured at 2, which is pinned
by a test.

Headless runs also read chunks=0 as unset (`or 2`), so an agent with
retrieval switched off retrieved anyway on scheduled runs; 0 now stays 0.

tests/api/answer/services/test_stream_processor_utils.py

@@ -473,7 +473,7 @@ class TestConfigureRetriever:
         sp = StreamProcessor({}, {"sub": "u"})
         sp._configure_retriever()
         assert sp.retriever_config["retriever_name"] == "classic"
-        assert sp.retriever_config["chunks"] == 2
+        assert sp.retriever_config["chunks"] == 6
 
     def test_agent_overrides(self):
         from docsgpt.api.answer.services.stream_processor import (
@@ -519,7 +519,7 @@ class TestConfigureRetriever:
         sp._agent_data = {}
         sp._configure_retriever()
         assert sp.retriever_config["retriever_name"] == "classic"
-        assert sp.retriever_config["chunks"] == 2
+        assert sp.retriever_config["chunks"] == 6
 
     def test_invalid_agent_chunks_falls_back(self):
         from docsgpt.api.answer.services.stream_processor import (
@@ -528,7 +528,7 @@ class TestConfigureRetriever:
         sp = StreamProcessor({}, {"sub": "u"})
         sp._agent_data = {"chunks": "not-a-number"}
         sp._configure_retriever()
-        assert sp.retriever_config["chunks"] == 2
+        assert sp.retriever_config["chunks"] == 6
 
     def test_invalid_request_chunks_falls_back(self):
         from docsgpt.api.answer.services.stream_processor import (
@@ -536,7 +536,7 @@ class TestConfigureRetriever:
         )
         sp = StreamProcessor({"chunks": "abc"}, {"sub": "u"})
         sp._configure_retriever()
-        assert sp.retriever_config["chunks"] == 2
+        assert sp.retriever_config["chunks"] == 6
 
     def test_isnonedoc_without_api_key_sets_chunks_to_0(self):
         from docsgpt.api.answer.services.stream_processor import (