chore: update launchdarkly-server-sdk-ai to ^0.15.0 and fix CI Python version

[kinyoklion]

Summary

Bumps the launchdarkly-server-sdk-ai dependency from ^0.14.0 (which resolves to >=0.14.0, <0.15.0) to ^0.15.0 (>=0.15.0, <0.16.0) to track the latest release of the Python AI SDK.

No other dependency or code changes are included.

Updates since last revision

Fixed CI failure by updating the Python version used in the CI workflow from 3.8 to 3.10. The pyproject.toml already required python = "^3.10", so CI was broken before this PR as well. Also updated the README to say "Python 3.10 or higher" to match the actual requirement.

Review & Testing Checklist for Human

• Verify that launchdarkly-server-sdk-ai 0.15.0 is API-compatible with the example code. Note: The examples currently import AIConfig from ldai.client, which does not exist in 0.14.x or 0.15.x (it appears to have been renamed to AICompletionConfig). This is a pre-existing issue, not introduced by this PR, but means the examples are broken on both old and new versions.
• Confirm the CI Python version bump from 3.8 → 3.10 is correct (it matches the existing python = "^3.10" constraint in pyproject.toml).
• Run poetry install --all-extras && poetry run openai-example (with appropriate env vars) to confirm the examples work after fixing the pre-existing import issue.

Notes

• The poetry.lock file is in .gitignore for this repo, so no lock file changes are included.
• The CI and README Python version fixes address a pre-existing mismatch — pyproject.toml already required ^3.10 but CI used 3.8 and the README said 3.8.
• Link to Devin Session: https://app.devin.ai/sessions/8b0a4069b434416dbaa05daac9bd26f9
• Requested by: @kinyoklion

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