Support headless offering checkpoints (#3951)

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Some checkpoint workflows only select an offering and leave presentation
to the app. This inspects the matched workflow and returns
`CheckpointResult.ReceivedOffering` when the workflow consists of a
single valid offering step.

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> [!NOTE]
> **Medium Risk**
> Changes checkpoint resolution and purchase/paywall entry paths (new
branch, renamed resolution types) with broad test coverage; behavior for
invalid configs is preserved but resolution order and dependency loading
differ.
> 
> **Overview**
> Checkpoints can now resolve to **app-owned offerings** instead of
always launching RevenueCat UI. When the winning workflow is a single
`offering` step with a valid `offering_identifier`, resolution returns
**`CheckpointResolution.MatchedOffering`** and the UI layer surfaces
**`CheckpointResult.ReceivedOffering`**—no Activity or one-at-a-time
presentation slot.
> 
> **UI workflows** are unchanged in behavior but renamed to
**`MatchedWorkflow`** (from `Workflow`). Resolution loads the workflow
body first via **`WorkflowManager.getWorkflowBody`**, branches on step
type, and only fetches workflow→offering mapping, offerings, and asset
prewarm for full UI paths. Invalid shapes (mixed steps, bad identifiers,
etc.) still fail the matched rule with **`CONFIGURATION_UNAVAILABLE`**
without falling through to later rules.
> 
> The checkpoint tester, API tester, and tests are updated for the new
result type and offering use case.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
6cc7ec7f9e6e5fa364115d876294f10dbe201459. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
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Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>