feat: ability to specify variation key on config or from_options for optimization package

[andrewklatzke]

Requirements

• I have added test coverage for new or changed functionality
• I have followed the repository's pull request submission guidelines
• I have validated my changes against all supported platform versions

Describe the solution you've provided

Adds the ability to specify a specific variation when setting up your configuration within LaunchDarkly, or specify a specific variation key in the from_options method as long as the API key is present. This allows a user to optimize against a specific variation rather than the default.

Describe alternatives you've considered

This is a relatively straightforward feature request. Allowing users to specify a specific variation cuts down on toil of managing your agents in the UI (don't need to change targeting or create a dummy config).

Additional context

This is purely additive. Functionality for configs not using the variation key, or that don't have it set, is unchanged -- it will continue using the default pulled via the SDK.

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Note

Medium Risk
Changes which agent instructions/model/tools seed an optimization run; wrong variation_key or API failures abort the run, but default behavior when unset is unchanged.

Overview
Adds variation_key support so optimization can start from a named LaunchDarkly AI config variation instead of the SDK’s context-evaluated default.

When variation_key is set (on OptimizationOptions / GroundTruthOptimizationOptions, or variationKey on the remote agent optimization config), _get_agent_config loads that variation through a new LDApiClient.get_ai_config_variation beta REST call and uses its instructions, tools, and model (modelConfigKey plus optional parameters). An optional reused api_client avoids extra clients in optimize_from_config. Missing variations surface LDApiError with no SDK fallback.

optimize_from_options, ground-truth options, and optimize_from_config forward the key and enforce API key + project_key when it is set. Tests cover the API client, agent config wiring, and entry points.

Note: dataclasses.py currently declares variation_key twice on both options types (duplicate field definitions in the diff)—worth cleaning before merge.

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