Expose LLM token usage across all service layers (#782)

Expose LLM token usage (in_token, out_token, model) across all
service layers

Propagate token counts from LLM services through the prompt,
text-completion, graph-RAG, document-RAG, and agent orchestrator
pipelines to the API gateway and Python SDK. All fields are Optional
— None means "not available", distinguishing from a real zero count.

Key changes:

- Schema: Add in_token/out_token/model to TextCompletionResponse,
  PromptResponse, GraphRagResponse, DocumentRagResponse,
  AgentResponse

- TextCompletionClient: New TextCompletionResult return type. Split
  into text_completion() (non-streaming) and
  text_completion_stream() (streaming with per-chunk handler
  callback)

- PromptClient: New PromptResult with response_type
  (text/json/jsonl), typed fields (text/object/objects), and token
  usage. All callers updated.

- RAG services: Accumulate token usage across all prompt calls
  (extract-concepts, edge-scoring, edge-reasoning,
  synthesis). Non-streaming path sends single combined response
  instead of chunk + end_of_session.

- Agent orchestrator: UsageTracker accumulates tokens across
  meta-router, pattern prompt calls, and react reasoning. Attached
  to end_of_dialog.

- Translators: Encode token fields when not None (is not None, not truthy)

- Python SDK: RAG and text-completion methods return
  TextCompletionResult (non-streaming) or RAGChunk/AgentAnswer with
  token fields (streaming)

- CLI: --show-usage flag on tg-invoke-llm, tg-invoke-prompt,
  tg-invoke-graph-rag, tg-invoke-document-rag, tg-invoke-agent

tests/integration/test_document_rag_streaming_integration.py

@@ -9,6 +9,7 @@ import pytest
 from unittest.mock import AsyncMock
 from trustgraph.retrieval.document_rag.document_rag import DocumentRag
 from trustgraph.schema import ChunkMatch
+from trustgraph.base import PromptResult
 from tests.utils.streaming_assertions import (
     assert_streaming_chunks_valid,
     assert_callback_invoked,
@@ -74,12 +75,14 @@ class TestDocumentRagStreaming:
                     is_final = (i == len(chunks) - 1)
                     await chunk_callback(chunk, is_final)
 
-                return full_text
+                return PromptResult(response_type="text", text=full_text)
             else:
                 # Non-streaming response - same text
-                return full_text
+                return PromptResult(response_type="text", text=full_text)
 
         client.document_prompt.side_effect = document_prompt_side_effect
+        # Mock prompt() for extract-concepts call in DocumentRag
+        client.prompt.return_value = PromptResult(response_type="text", text="")
         return client
 
     @pytest.fixture
@@ -119,11 +122,12 @@ class TestDocumentRagStreaming:
         collector.verify_streaming_protocol()
 
         # Verify full response matches concatenated chunks
+        result_text, usage = result
         full_from_chunks = collector.get_full_text()
-        assert result == full_from_chunks
+        assert result_text == full_from_chunks
 
         # Verify content is reasonable
-        assert len(result) > 0
+        assert len(result_text) > 0
 
     @pytest.mark.asyncio
     async def test_document_rag_streaming_vs_non_streaming(self, document_rag_streaming):
@@ -159,9 +163,11 @@ class TestDocumentRagStreaming:
         )
 
         # Assert - Results should be equivalent
-        assert streaming_result == non_streaming_result
+        non_streaming_text, _ = non_streaming_result
+        streaming_text, _ = streaming_result
+        assert streaming_text == non_streaming_text
         assert len(streaming_chunks) > 0
-        assert "".join(streaming_chunks) == streaming_result
+        assert "".join(streaming_chunks) == streaming_text
 
     @pytest.mark.asyncio
     async def test_document_rag_streaming_callback_invocation(self, document_rag_streaming):
@@ -180,8 +186,9 @@ class TestDocumentRagStreaming:
         )
 
         # Assert
+        result_text, usage = result
         assert callback.call_count > 0
-        assert result is not None
+        assert result_text is not None
 
         # Verify all callback invocations had string arguments
         for call in callback.call_args_list:
@@ -202,7 +209,8 @@ class TestDocumentRagStreaming:
 
         # Assert - Should complete without error
         assert result is not None
-        assert isinstance(result, str)
+        result_text, usage = result
+        assert isinstance(result_text, str)
 
     @pytest.mark.asyncio
     async def test_document_rag_streaming_with_no_documents(self, document_rag_streaming,
@@ -223,7 +231,8 @@ class TestDocumentRagStreaming:
         )
 
         # Assert - Should still produce streamed response
-        assert result is not None
+        result_text, usage = result
+        assert result_text is not None
         assert callback.call_count > 0
 
     @pytest.mark.asyncio
@@ -271,7 +280,8 @@ class TestDocumentRagStreaming:
             )
 
             # Assert
-            assert result is not None
+            result_text, usage = result
+            assert result_text is not None
             assert callback.call_count > 0
 
             # Verify doc_limit was passed correctly