feat: add getQuery method to Firestore transactions for transaction…

[kartikey321]

Summary

• add Transaction.getQuery to let queries participate in Firestore transactions with pessimistic locking (lazy
  transaction start via newTransaction on RunQuery)
• capture transaction IDs from RunQueryResponse via new _QueryReader, mirroring _DocumentReader
• bump package to 0.4.2, add changelog entry, and ship 10 new query-transaction tests covering mixed reads,
  read-only, limits, converters, and read-after-write enforcement

Testing

• dart test test/google_cloud_firestore/transaction_test.dart (Firestore emulator running)

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