GenAI utils | Create Agent demo app

[etserend]

Description

Adds AgentCreation type and create_agent() context manager to opentelemetry-util-genai, enabling instrumentation of agent creation and deployment lifecycle events with OpenTelemetry spans following the GenAI agent semantic conventions.

The create_agent span captures agent identity, provider, model, and server metadata as span attributes. This is the first of a series of PRs adding agent lifecycle telemetry to the shared GenAI utilities package.

Example span output (deploying a LanggraphAgent to Vertex AI Agent Engine):

Trace (95a94efb81565d932cd55f8a358b6a48)
└── create_agent Currency Exchange Agent (4m5.24s, Client)
    ├── gen_ai.agent.description: Deploying agent to Vertex AI Agent Engine
    ├── gen_ai.agent.name: Currency Exchange Agent
    ├── gen_ai.operation.name: create_agent
    ├── gen_ai.provider.name: gcp_vertex_ai
    ├── gen_ai.request.model: gemini-2.5-flash
    └── server.address: us-central1-aiplatform.googleapis.com

Fixes # (issue)

Type of change

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• New feature (non-breaking change which adds functionality)