feat: stream intermediate assistant turns over MCP

Install a per-request `after_llm_call` hook that emits each intermediate
assistant turn as an MCP `notifications/message`, so users see
substantive text from earlier loop iterations instead of only the final
`agent.send()` return value.

Add tests covering the hook's payload shape, error handling, and
lifecycle via `install_for_request`.

pallas/multimodal_server.py

@@ -28,6 +28,7 @@ from fast_agent.core.logging.logger import get_logger
 from fast_agent.mcp.server import AgentMCPServer
 from fast_agent.types import PromptMessageExtended, RequestParams
 
+from pallas.assistant_stream import install_for_request as _install_assistant_stream
 from pallas.progress import EnrichedMCPToolProgressManager
 from pallas import metrics as _pallas_metrics
 from fastmcp import Context as MCPContext
@@ -220,6 +221,20 @@ class MultimodalAgentMCPServer(AgentMCPServer):
             agent_context = getattr(agent, "context", None)
             metrics_start = time.perf_counter()
             metrics_outcome = "ok"
+
+            # Install per-request after_llm_call hook that ships every
+            # intermediate assistant turn over MCP as a notifications/message.
+            # Without this, only the final ``agent.send()`` return value
+            # crosses the MCP boundary — substantive assistant text emitted
+            # in earlier loop iterations stays trapped inside fast-agent's
+            # ``message_history`` and the user sees a spinner that ends with
+            # a thin wrap-up sentence.
+            restore_hooks = _install_assistant_stream(
+                agent,
+                ctx=ctx,
+                agent_name=agent_name,
+                conversation_id=conversation_id,
+            )
             try:
                 # Seed the freshly-created instance's message_history from the
                 # caller-supplied history so the agent sees the full
@@ -313,6 +328,14 @@ class MultimodalAgentMCPServer(AgentMCPServer):
                 _pallas_metrics.send_message_total.labels(
                     agent=agent_name, outcome=metrics_outcome
                 ).inc()
+                # Restore the agent's prior tool_runner_hooks before the
+                # instance is released — defensive against any future
+                # shared-instance mode where leaking per-request hooks
+                # across requests would mis-attribute notifications.
+                try:
+                    restore_hooks()
+                except Exception:
+                    pass
                 await self._release_instance(ctx, instance)