fix: Specify ld-ai context kind for SDK info tracking

[jsonbailey]

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

Related issues

• Implements AIC-1636
• Spec change: https://github.com/launchdarkly/sdk-specs/pull/134
• Follow-up to fix: Improve usage reporting #92

Describe the solution you've provided

Sets the context kind to ld-ai on the anonymous context used for the $ld:ai:sdk-info tracking event, per the updated spec in sdk-specs#134. Previously the context used the default user kind.

Human review checklist

• Verify the kind string ld-ai matches the spec exactly
• The existing test_sdk_info_tracked_on_init test imports _INIT_TRACK_CONTEXT by reference rather than constructing an expected context independently — it will pass but does not explicitly assert the kind is ld-ai. Consider whether a more explicit assertion is warranted.

Additional context

• Link to Devin run: https://app.devin.ai/sessions/6c2677b974304088b7bef430002d9884
• Requested by: @jsonbailey

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