chore: update launchdarkly_server_sdk lock to 3.9.0

[kinyoklion]

Summary

Updates mix.lock to resolve launchdarkly_server_sdk to 3.9.0 (was 3.5.0). The version spec in mix.exs (~> 3.5) already permits this; only the lock file was stale.

Transitive dependency updates pulled in by the SDK bump:

• certifi 2.12.0 → 2.16.0
• cowlib 2.13.0 → 2.16.0
• gun 2.1.0 → 2.2.0
• shotgun 1.1.0 → 1.2.1

Review & Testing Checklist for Human

• Run mix deps.get && mix compile to verify the project compiles cleanly with the updated lock
• Run the example app (mix run) with a valid SDK key and confirm flag evaluation still works
• Spot-check that gun 2.2.0 / cowlib 2.16.0 don't introduce issues with the SDK's streaming connection

Notes

• No application code changes — this is purely a lock file refresh.
• The mix.exs version constraint (~> 3.5) was not modified; it already allows 3.9.0.

Link to Devin session: https://app.devin.ai/sessions/10bc3fe216ca49f1839e06a9c2bee629
Requested by: @kinyoklion

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