bug: expose agent_framework and labels in ReasoningEngine.create

[FreddyAyala]

Fixes #6267

Description

This PR updates ReasoningEngine.create (and _prepare) to accept agent_framework, labels, and env_vars. These parameters are passed down to the underlying ReasoningEngine resource creation.

Motivation

Currently, deploying an ADK-based agent (google-adk) via the Python SDK results in an agent that is missing critical metadata in the Vertex AI Console. Specifically:

• Framework: Shows as "Custom" instead of "google-adk".
• Labels: Missing the required goog-vertex-reasoning-engine-framework: adk label.
• Telemetry: Cannot inject OTEL_... environment variables into the deployment spec.

without this metadata, the "Traces", "Sessions", and "Dashboards" tabs in the Vertex AI Console are disabled. This change allows users to set these values directly during creation, enabling full Console feature parity with Terraform deployments.

Changes

• Updated ReasoningEngine.create() signature to include agent_framework, labels, and env_vars.
• Updated _reasoning_engines._prepare to propagate these values.
• Validated that env_vars are correctly converted to aip_types.EnvVar protos.
• Added unit test test_create_reasoning_engine_with_metadata in tests/unit/vertex_langchain/test_reasoning_engines.py to verify parameter passing.