fix(firestore)!: Make DocumentSnapshot.exists() a method rather than a property (BREAKING CHANGE)

[MichaelVerdon]

Description

closes #8460

Just as @mikehardy says, JS documentation states exists() is a method returning true or false.
https://firebase.google.com/docs/reference/js/database.datasnapshot.md#datasnapshotexists
Very small breaking change, users just need to change snapshot.exists to snapshot.exists().

To be shipped with other breaking changes.

Related issues

Release Summary

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
  • Yes
• My change supports the following platforms;
  • Android
  • iOS
  • Other (macOS, web)
• My change includes tests;
  • e2e tests added or updated in packages/\*\*/e2e
  • jest tests added or updated in packages/\*\*/__tests__
• I have updated TypeScript types that are affected by my change.
• This is a breaking change;
  • Yes
  • No

Test Plan

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Name	Status	Preview	Comments	Updated (UTC)
react-native-firebase	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Apr 23, 2025 9:27am

[mikehardy]

excellent

[MorganTrudeau]

I think firestore/lib/index.d.ts needs to updated with this change right?

[mikehardy]

@MorganTrudeau great catch - you are correct