Rename `session_id` to `app_session_id` in checkpoint events (#4039)

Equivalent iOS https://github.com/RevenueCat/purchases-ios/pull/7459
Khepri PR https://github.com/RevenueCat/khepri/pull/24141

Rename `session_id` to `app_session_id` in checkpoint events


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---

> [!NOTE]
> **Medium Risk**
> Changes the wire schema for checkpoint events; backend and Khepri must
accept `app_session_id` before or with this SDK release to avoid dropped
or mis-parsed checkpoint data.
> 
> **Overview**
> Renames the checkpoint backend event field from **`session_id`** to
**`app_session_id`** on `BackendEvent.Checkpoint`, with the Kotlin
property **`sessionID`** → **`appSessionID`**.
> 
> The **`CheckpointEvent.toBackendStoredEvent`** mapper takes
**`appSessionID`** instead of **`sessionID`**, so serialized checkpoint
payloads and persisted event lines use the new key. Tests for
khepri-compatible JSON and **`EventsManager`** checkpoint file output
were updated accordingly.
> 
> Paywall events still use **`session_id`**; only checkpoint events
change in this PR (aligned with iOS and Khepri).
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
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<!-- /CURSOR_SUMMARY -->