fix(firestore, web): update Timestamp handling in jsify and EncodeUtility

[SelaseKay]

Description

Problem

On web, startAfter / startAfterDocument could return the same document again when the result set had only one matching row, causing infinite pagination loops.

Cause

Cursor values for web queries are built from encoded field values. EncodeUtility.valueEncode converted Dart Timestamp to DateTime via toDate(), which loses sub-millisecond precision when passed to the Firestore JS SDK. The cursor tuple could then sort before the real document position in the index, so the next page still included that document.

Fix

Encode Dart Timestamp as the Firestore JS Timestamp (seconds + nanoseconds) in EncodeUtility.valueEncode, matching how the SDK compares stored timestamps.

Related Issues

Fixes #18233

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