…resentation control

Replace hardcoded full-screen presentation with an explicit opt-in API,
matching the approach in purchases-unity PR #839. Developers can now
choose full-screen or sheet presentation per platform instead of having
it forced.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

…tation style (#1623)

## Summary

Fixes landscape white-area issue (PW-148) by adding an explicit opt-in
API for controlling paywall presentation style, matching the approach in
[purchases-unity PR
#839](RevenueCat/purchases-unity#839).

- Adds \`PaywallPresentationConfiguration\` with per-platform style
types (\`IOSPaywallPresentationStyle\`,
\`AndroidPaywallPresentationStyle\`)
- Adds optional \`presentationConfiguration\` parameter to
\`presentPaywall()\` and \`presentPaywallIfNeeded()\`
- iOS plugin passes \`useFullScreenPresentation\` only when explicitly
set — default behavior unchanged (sheet on iOS)
- No device filtering — if the dev passes \`fullScreen\`, it applies on
any device (iPhone or iPad)
- Purchase tester shows a \`CupertinoActionSheet\` to choose Sheet or
Full Screen before presenting

### Usage

```dart
// Full screen on iOS (fixes landscape white-area bug PW-148)
final result = await RevenueCatUI.presentPaywall(
  presentationConfiguration: PaywallPresentationConfiguration(
    ios: IOSPaywallPresentationStyle.fullScreen,
  ),
);

// Default behavior (sheet on iOS)
final result = await RevenueCatUI.presentPaywall();
```

### Related

- Unity: RevenueCat/purchases-unity#839
- Linear: PW-148, PW-956

## Test plan

- [x] All unit tests pass
- [x] API tester (\`flutter analyze\`) clean — no breaking changes
- [x] Tests cover \`fullScreen\` and \`sheet\` configurations
- [x] Tests verify \`useFullScreenPresentation\` is not sent when no
config is passed
- [x] Purchase tester builds on iOS
- [x] Manual test: present paywall without config → sheet on iOS
- [x] Manual test: present paywall with \`ios: fullScreen\` → full
screen on iOS

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Medium Risk**
> Adds a new public API knob that changes iOS paywall presentation
behavior and updates the iOS plugin call path; incorrect wiring could
affect paywall UX on iOS. Changes are scoped and covered by new unit/API
tests, reducing regression risk.
> 
> **Overview**
> Adds an opt-in `PaywallPresentationConfiguration` API (with
`IOSPaywallPresentationStyle`/`AndroidPaywallPresentationStyle`) and
exports it from `purchases_ui_flutter`.
> 
> `RevenueCatUI.presentPaywall()` and `presentPaywallIfNeeded()` now
accept an optional `presentationConfiguration` and only send the native
`useFullScreenPresentation` flag when iOS full-screen is explicitly
requested (omitting the key preserves the native default sheet
behavior).
> 
> Updates the iOS plugin to accept/pass through
`useFullScreenPresentation` instead of always forcing full-screen on
iPhone, expands unit tests/API tester coverage for the new argument
behavior, and updates the purchase tester example UI to let users choose
sheet vs full-screen before presenting a paywall.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
35fb97d. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>
Co-authored-by: Cesar de la Vega <664544+vegaro@users.noreply.github.com>