Update a cross-project Holdout

Update a holdout experiment using JSON Patch (RFC 6902) operations. Supports status transitions, configuration updates, and target changes.

Authorization:

• Requires HOLDOUTS_ACCESS permission (feature-level gate)
• Requires FLAGS_ADMIN permission on ALL affected projects (both old and new targets)
• Gated behind holdouts-m2 feature flag

Optimistic Locking:

• Include If-Match header with current revision ETag to prevent concurrent updates
• Server returns 409 Conflict if holdout was modified since client's last read

Supported Operations:

• Update name, description, traffic allocation (any status)
• Transition status: draft → running → concluded
• Update scope (draft only)
• Update project_ids for global holdouts (draft only)
• Update rule_ids for local holdouts (draft only)
• Update metrics
• Update audience_conditions

Validation Rules:

• Scope changes only allowed in draft status
• Target changes (project_ids/rule_ids) only allowed in draft status
• Cannot update project_ids on local holdouts (400 error)
• Cannot update rule_ids on global holdouts (400 error)
• Status transitions must be valid (draft→running, running→concluded)
• Traffic allocation must be 0-10000 basis points
• Cannot set audience_conditions on a holdout whose targeted rules span more than one project (400 error)