feat(core): Implement AgentRuntime with metadata and state tracking

[ziye122]

… #33880

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Description

Add AgentRuntime to manage agent metadata (via AgentMetadata) and runtime state (via AgentState), analogous to ToolRuntime. This includes:

• Immutable metadata (name, model, type, etc.) with serialization support.
• Dynamic state tracking (call count, status, errors) with lifecycle hooks (on_call/on_complete).
• Unit tests covering core logic (metadata serialization, state updates, lifecycle).

Issue

Closes #33880

Dependencies

None