Add the duo_developer_continuable_sessions feature flag

What does this MR do and why?

This MR adds the wip feature flag duo_developer_continuable_sessions and a flow_version_resolver on the Developer foundational flow that reads it. When the flag is on for the user or the container's top-level group, it selects developer/4.0.0. That version rests after each turn instead of ending, so people can follow up in the same session (gitlab-org#24022, rollout: #633281). It is the first Rails step of the continuable-sessions stack: with the flag off (the default), the resolver returns nil and sessions keep today's ^3.0.0.

For reviewers: developer/4.0.0 doesn't exist in the AI Gateway yet and follows in gitlab-org/modelops/applied-ml/code-suggestions/ai-assist#3173; this flag is one step of that stack, and #633281 tracks when it gets enabled.

Existing specs changed to stub the flag off, because they describe today's behaviour:

• ee/spec/lib/ai/duo_workflows/foundational_flow_start_params_resolver_spec.rb
• ee/spec/models/ai/catalog/foundational_flow/developer/definition_spec.rb (existing examples; the new examples are in the same file)

References

• Related to #633281
• Epic: gitlab-org#24022
• AI Gateway flow version: gitlab-org/modelops/applied-ml/code-suggestions/ai-assist#3173

Screenshots or screen recordings

Not applicable: no UI change.

How to set up and validate locally

How to test: in a Rails console (gdk rails c):

flow = Ai::Catalog::FoundationalFlow.developer_v1
group = Group.top_level.first
project = group.all_projects.first
user = User.first
other_user = User.where.not(id: user.id).first

# Flag off (default) -> "^3.0.0"
flow.resolve_flow_version_for(container: project, user: user)[:flow_version]

# Flag on for a user -> "^4.0.0"
Feature.enable(:duo_developer_continuable_sessions, user)
flow.resolve_flow_version_for(container: project, user: user)[:flow_version]
Feature.disable(:duo_developer_continuable_sessions, user)

# Flag on for the top-level group -> "^4.0.0" for anyone in a project below it
Feature.enable(:duo_developer_continuable_sessions, group)
flow.resolve_flow_version_for(container: project, user: other_user)[:flow_version]
# No container -> "^3.0.0", no error
flow.resolve_flow_version_for(container: nil, user: other_user)[:flow_version]
Feature.disable(:duo_developer_continuable_sessions, group)

# Back to "^3.0.0"
flow.resolve_flow_version_for(container: project, user: user)[:flow_version]

Not in this MR

Everything else in gitlab-org#24022, including:

• the developer/4.0.0 flow version in the AI Gateway (gitlab-org/modelops/applied-ml/code-suggestions/ai-assist#3173)
• any idle or "resting" session status, and follow-up (continue session) handling in Rails
• API and frontend changes
• enabling the flag in any environment (tracked in #633281)

MR acceptance checklist

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