Add run configuration inheritance for agent-as-tool workflows

[DanielHashmi]

Problem:
When using the "agents as tools" pattern, tool-agents create their own execution context but don't inherit the parent's run_config.

This creates two problems:
Tool-agents can't access the parent's global model configuration
If tool-agents don't have their own model field set and the parent's run_config.model is unavailable, model resolution fails entirely, causing the orchestrator to fail when calling the tool

Solution:
Modified the agent-as-tool execution to properly handle run_config inheritance while ensuring robust model resolution. This prevents execution failures when tool-agents lack explicit model configuration.

Changes

1. traces.py

• Added _run_config attribute to both NoOpTrace and TraceImpl classes
• Added TYPE_CHECKING import for RunConfig type annotation
• Enables storing run configuration in trace context for cross-agent access

2. run.py

• Enhanced trace context management to store run configuration
• Implements two-tier storage: metadata dictionary (preferred) or _run_config attribute (fallback)
• Ensures sub-agents can access original run configuration during execution

3. agent.py

• Added get_current_trace import from tracing module
• Enhanced as_tool() method to retrieve and inherit run configuration from trace context
• Passes inherited configuration to Runner.run() for consistent behavior