.NET Workflows - Declarative context/thread reset support for iterative agent invocations

## Description

### Problem

In declarative workflows, when using multiple agents and iterating over a list of inputs (e.g., creating summaries for each item), there's currently no way to reset the agent's context/thread state between iterations.

**Use Case Example:**
```yaml
actions:
- kind: Foreach
items: =workflow.input.documents
actions:
- kind: InvokeAzureAgent
agent: summarizer
input:
messages:
- =loop.item.content
output:
path: turn.summaries
append: true
```

In this scenario, when the summarizer agent processes each document, the conversation history from previous iterations accumulates. This causes:
- Context pollution between unrelated documents
- Increased token usage as history grows
- Potential confusion for the agent model

### Expected Behavior

The ability to reset an agent's thread/context state within declarative workflows:

1. **Per-invocation option** - A `resetThread` flag on `InvokeAzureAgent`:
```yaml
- kind: InvokeAzureAgent
agent: summarizer
resetThread: true
```

2. **Explicit action** - A new `ResetAgentContext` action kind:
```yaml
- kind: ResetAgentContext
agent: summarizer
```

3. **Via BaseAgent** - Expose thread reset methods that declarative workflows can invoke (similar to how `MagenticAgentExecutor` handles `MagenticResetSignal`)

### Alternatives Considered

- Creating new agent instances per iteration (inefficient)
- Manual thread management in code (not available in declarative/YAML workflows)

### Related Code

The framework already has reset patterns:
- `MagenticAgentExecutor.handle_magentic_reset()` → `self._agent_thread = self._agent.get_new_thread()`
- `AgentExecutor.reset()` clears internal cache
- `BaseGroupChatOrchestrator._clear_conversation()`
- `AgentEntity.reset()` in durable task entities

## Code Sample

```yaml
# Option 1: Per-invocation reset
- kind: Foreach
items: =workflow.input.documents
actions:
- kind: InvokeAzureAgent
agent: summarizer
resetThread: true
input:
messages:
- =loop.item.content

# Option 2: Explicit reset action
- kind: Foreach
items: =workflow.input.documents
actions:
- kind: ResetAgentContext
agent: summarizer
- kind: InvokeAzureAgent
agent: summarizer
input:
messages:
- =loop.item.content
```

## Language/SDK

Python

Start by tracing declarative workflow handling for InvokeAzureAgent, then compare the reset patterns in MagenticAgentExecutor.handle_magentic_reset(), AgentExecutor.reset(), BaseGroupChatOrchestrator._clear_conversation(), and AgentEntity.reset(). Done means declarative workflows can reset agent thread or context between iterations using a clearly defined mechanism, with the documented examples behaving as expected.

Written by the indexing model from the issue text.