feat(firestore): add FieldValue.minimum and FieldValue.maximum

[SelaseKay]

Description

Adds FieldValue.minimum and FieldValue.maximum write sentinels so set() / update() can clamp a numeric field to the min or max of its current value and the given operand.

Native iOS, Android, and JS already expose these APIs. This PR wraps them in Dart, the platform interface codec (types 200–203), Apple/Android codecs, and web JS interop.

Windows is not supported: the Firebase C++ SDK does not expose FieldValue::Minimum / Maximum yet. The Windows codec consumes the payload and throws an explicit error. E2E tests for these sentinels are skipped on Windows.

Related Issues

No existing FlutterFire issue. Tracked as a missing_api in the 2026-09-11 Firebase SDK watcher report after iOS 12.19.0 added the native APIs.

Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See [Contributor Guide]).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (melos run analyze) does not report any problems on my PR.
• I read and followed the [Flutter Style Guide].
• I signed the [CLA].
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change.
• No, this is not a breaking change.

Test plan

• Unit tests in cloud_firestore/test/field_value_test.dart
• Web factory tests in cloud_firestore_web/test/field_value_factory_web_test.dart
• E2E in example/integration_test/field_value_e2e.dart on iOS, Android, and web: min/max vs existing value, missing field, non-numeric field
• Confirm Windows throws a clear unsupported error (or the e2e skip holds)

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