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)

tests/client-sdk/agents/test_agents.py

@@ -19,8 +19,12 @@ from llama_stack_client.types.shared.completion_message import CompletionMessage
 from llama_stack_client.types.shared_params.agent_config import AgentConfig, ToolConfig
 from llama_stack_client.types.tool_def_param import Parameter
 
-from llama_stack.apis.agents.agents import AgentConfig as Server__AgentConfig
-from llama_stack.apis.agents.agents import ToolChoice
+from llama_stack.apis.agents.agents import (
+    AgentConfig as Server__AgentConfig,
+)
+from llama_stack.apis.agents.agents import (
+    ToolChoice,
+)
 
 
 class TestClientTool(ClientTool):