fix(experiment): drop removed scope and enrollment fields

[manueltollis]

The OpenAPI client update on openapi-generation-main (2026.09.29.1117-e0b6f6d) removed scope, eligibility and userEnrollment from CreateExperimentEndpointInput. This broke the build: pyright found 2 errors in experiment_manager.py and test_create_posts_attached_scope_then_activates_by_id failed.

• ExperimentManager.create no longer sends scope / userEnrollment, and its user_enrollment parameter is removed (it's internal and has no other callers).
• The test no longer checks scope.

Verified: pyright src/rapidata/rapidata_client has 0 errors, and pytest shows 324 passed.

Agent skill: unchanged. ExperimentManager is internal and the skill doesn't document it.

🔗 Session: https://poseidon.rapidata.internal/chat/session-083ebd9e

🤖 Generated with Claude Code

[github-actions]

✅ No skill update needed — confirmed by @LinoGiger

This PR does not modify src/rapidata/_skill/. That directory is the agent skill that ships in every SDK release and that coding agents read through python -m rapidata skill.

Before merging, pick one:

1. The change affects what an agent needs to know (new or renamed API, changed parameter, default or result field, new gotcha): update src/rapidata/_skill/SKILL.md (or a companion guide next to it) in this PR.
2. Nothing the skill documents changed: a reviewer applies the skill-unchanged-approved label. New commits remove the label again.

[RapidPoseidon]

CI note: the only red check is the Agent Skill gate. It is waiting on a reviewer decision, not a code failure. This PR changes only the internal ExperimentManager, which src/rapidata/_skill/ doesn't document, so the skill needs no update. A reviewer can apply skill-unchanged-approved to clear it.

Tests and pyright only trigger on PRs into main/2.x, so they don't run against this base (openapi-generation-main). Locally: pyright has 0 errors, and pytest shows 324 passed.

[RapidPoseidon]

@LinoGiger thanks for the approval. The Agent Skill status is still red because skill-unchanged-approved isn't on the PR. The change only touches the internal ExperimentManager, which the skill doesn't document. Could you add the label? I'll merge once it's green.