feat: add reward ad show API

[2sem]

Goal and success criteria

• Support developer-facing rewarded ad flow via prepare(rewardUnit:) and show(rewardUnit:).
• Return GoogleMobileAds.RewardedAd? directly from the reward-specific show callback.
• Expose reward amount and type through rewardedAd.adReward.
• Preserve existing generic show(unit:) compatibility.

User flow

flowchart TD
    A[App calls prepare rewardUnit] --> B[GADManager loads RewardedAd]
    B -->|Success| C[Cache rewarded ad]
    B -->|Failure| F[Completion false]
    C --> D[App calls show rewardUnit]
    D --> E[Present rewarded ad]
    E --> G{User earned reward?}
    G -->|Yes| H[Callback RewardedAd + rewarded=true]
    G -->|No| I[Callback rewarded=false]
    H --> J[Read adReward amount and type]
    J --> K[Auto reprepare]
    I --> K

Loading

Workstreams and deliverables

• Library API: add show(rewardUnit:...) returning RewardedAd? and improve load failure handling.
• Lifecycle: preserve test mode, force, and presenting view controller while waiting for a reward ad to load.
• Lifecycle: retain rewarded ad reprepare behavior and test label configuration.
• Docs: update README rewarded ad example.

Dependencies and critical path

• Depends on the existing GADManager rewarded ad lifecycle and GoogleMobileAds RewardedAd API.
• Critical path: load rewarded ad -> present -> earn reward callback -> dismissal completion -> reprepare.

Risks and mitigations

• Backward compatibility: existing prepare(rewardUnit:isTesting:) calls remain valid because the new parameter has a default.
• Hanging callbacks: load failure and missing ad ID clear and invoke pending completions with false.
• Deferred presentation: test mode and supplied presentation options are retained across loading.
• Reward metadata: callers receive RewardedAd and can read adReward.amount and adReward.type.

Verification

• git diff --check origin/main passes.
• swift build --triple arm64-apple-ios12.0-simulator --sdk "$(xcrun --sdk iphonesimulator --show-sdk-path)" passes.
• No automated test target exists in the package.

Release checklist

• Validate rewarded presentation in a host app.
• Confirm RewardedAd.adReward returns the expected amount and type.
• Confirm rewarded is true only after reward is earned.
• Monitor ad load and presentation failures after release.
• Roll back by reverting this PR if reward integration regresses.

[github-actions]

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