feat(firestore): add support for withConverter on Writebatch

[Lyokone]

Description

WriteBatch.update did not support typed data from withConverter, unlike WriteBatch.set. This updates the Dart batch API to convert non-map typed data with the document reference converter while preserving existing map-based partial updates, including FieldPath keys. The regression coverage includes both a simple converted value and a nested model object.

Related Issues

• Fixes 🐛 [firestore] BatchWrite.update does not implement type-safe read & write operations. #11552

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

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

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