feat(1/n): api: unify agents for handling server & client tools (#1178)

# Problem

Our current Agent framework has discrepancies in definition on how we
handle server side and client side tools.

1. Server Tools: a single Turn is returned including `ToolExecutionStep`
in agenst
2. Client Tools: `create_agent_turn` is called in loop with client agent
lib yielding the agent chunk

ad6ffc63df/src/llama_stack_client/lib/agents/agent.py (L186-L211)

This makes it inconsistent to work with server & client tools. It also
complicates the logs to telemetry to get information about agents turn /
history for observability.

#### Principle
The same `turn_id` should be used to represent the steps required to
complete a user message including client tools.

## Solution

1. `AgentTurnResponseEventType.turn_awaiting_input` status to indicate
that the current turn is not completed, and awaiting tool input
2. `continue_agent_turn` endpoint to update agent turn with client's
tool response.


# What does this PR do?
- Skeleton API as example

## Test Plan
[Describe the tests you ran to verify your changes with result
summaries. *Provide clear instructions so the plan can be easily
re-executed.*]

- Just API update, no functionality change
```
llama stack run + client-sdk test
```

<img width="842" alt="image"
src="https://github.com/user-attachments/assets/7ac56b5f-f424-4632-9476-7e0f57555bc3"
/>


[//]: # (## Documentation)

llama_stack/providers/inline/agents/meta_reference/persistence.py

@@ -12,7 +12,7 @@ from typing import List, Optional
 
 from pydantic import BaseModel
 
-from llama_stack.apis.agents import Turn
+from llama_stack.apis.agents import ToolExecutionStep, Turn
 from llama_stack.providers.utils.kvstore import KVStore
 
 log = logging.getLogger(__name__)
@@ -84,3 +84,15 @@ class AgentPersistence:
                 continue
         turns.sort(key=lambda x: (x.completed_at or datetime.min))
         return turns
+
+    async def set_in_progress_tool_call_step(self, session_id: str, turn_id: str, step: ToolExecutionStep):
+        await self.kvstore.set(
+            key=f"in_progress_tool_call_step:{self.agent_id}:{session_id}:{turn_id}",
+            value=step.model_dump_json(),
+        )
+
+    async def get_in_progress_tool_call_step(self, session_id: str, turn_id: str) -> Optional[ToolExecutionStep]:
+        value = await self.kvstore.get(
+            key=f"in_progress_tool_call_step:{self.agent_id}:{session_id}:{turn_id}",
+        )
+        return ToolExecutionStep(**json.loads(value)) if value else None