Bump Duo Developer foundational flow config to ^3.0.0

What does this MR do and why?

Points the Duo Developer foundational flow at flow config version ^3.0.0 (caret range, so minor/patch releases within the 3.x line are picked up automatically).

Do not merge until the ai-assist 3.0.0 flow config MR is released. This Rails change must only land after the corresponding 3.0.0 config is published in the ai-assist repo, so that end-to-end testing can be completed first.

The 2.0.0-interactive value in Ai::FlowsMetadataService is a separate, out-of-scope variant and is intentionally left unchanged.

References

• Tracking task: #617752 (closed)

Screenshots or screen recordings

N/A - no UI change.

How to set up and validate locally

1. To test this properly you need the ai gateway side change here gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!6567 (merged)
2. Checkout both branches, restart your GDK and then try to trigger a developer flow (let it create a merge request)
3. The flow being used should be the new 3.0.0 version and it should finish a coding tasks without any issues

MR acceptance checklist

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