Add Braintrust telemetry exporter

[Innex]

Description

Adds a new telemetry exporter for Braintrust, enabling NAT workflows to send traces for AI observability and evaluation.

Key changes:

• New BraintrustTelemetryExporter class with OTEL support following existing exporter patterns
• Attribute mapping from OpenInference to GenAI semantic conventions (gen_ai.prompt, gen_ai.completion, gen_ai.usage.*)
• Span type classification mapping OpenInference kinds (LLM, CHAIN, TOOL, etc.) to Braintrust types
• Improved span naming for better Braintrust UI display
• Redundant attribute cleanup to reduce metadata clutter
• Step-by-step documentation guide
• Example YAML configuration
• 32 unit tests

Usage:

general:
  telemetry:
    tracing:
      braintrust:
        _type: braintrust
        project: my-project

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• When the PR is ready for review, new or existing tests cover these changes.
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