chore: promote SDK changes from staging

[borisbarath-stripe]

Automated promotion from staging, re-audited against the live tree diff.

What is actually landing

API surface

• Adds V2 lifecycle notification resources: offset notification create/get/list/archive/edit and system event listing.
• Adds billable-metric name updates.
• Adds custom pricing-unit create and archive methods.
• Adds required anchor_date fields to recurring commit and credit response models.
• Exposes subscription-product product_custom_fields and subscription-name edits.
• Adds duration, rate_type, and rollover_fraction to prepaid-balance threshold commit configuration.
• Adds uniqueness keys for manual balance entries and notification creation.
• Adds invoice line-item quantity_consumed and clarifies billable_status.
• Adds the generated body-cursor pagination helper and corresponding RBI/RBS types.

SDK automation and release metadata

• Hardens promotion/sync against stale runs and adds newest-run-wins concurrency.
• Uses STLC App authentication for production promotion operations.
• Restricts release and RubyGems publishing jobs to the production Ruby repository.
• Keeps release-PR descriptions synchronized for in-place version overrides.
• Removes the retired Stainless artifact-upload job and unrelated MCP README section.
• Changes the checked-in version representation from 3.11.0-alpha.1 to 3.11.0.pre.alpha.1.

Review notes

This is a large accumulated promotion. Confirm the prerelease version normalization before merging; it changes README, Gemfile.lock, and MetronomeSDK::VERSION.

Freshness and validation

• Promotion head exactly matches staging main.
• Required lint and test checks pass.

Diff reviewed

• 152 files, +6,073 / -119
• Production base: 2e7b436
• Staging head: a690aa4

Merge with a merge commit (not squash or rebase) to preserve staging/production ancestry.