fix(database,windows): fix transaction threwing and native error codes dropped

[Lyokone]

Description

Fixes two firebase_database bugs on Windows.

Transaction aborts threw instead of resolving with committed: false. The desktop C++ SDK never emits kErrorTransactionAbortedByUser, so the plugin's benign branch was dead code there. It reports a deliberate abort as kErrorWriteCanceled on the handler's initial invocation, and (because Error error = kErrorNone is declared and never assigned) as kErrorNone on a rerun, which the plugin reported as committed: true. Neither code can be trusted alone: kErrorWriteCanceled also means "cancelled by PurgeOutstandingWrites()" and must stay an error. The transaction context now records the Dart handler's own decision and the data it saw, and the completion handler keys the contract off that.

Native error codes were dropped, so every failure surfaced as unknown. ParseError() and both listener OnCancelled paths used the two-argument FlutterError/EventSink::Error overloads, which send null Pigeon details, and the shared converter read the code only from details. Now the Windows plugin sends a details map keyed code/message (as Android does), and platformExceptionToFirebaseException falls back to PlatformException.code when there is no details payload, which also recovers codes for firebase_storage and firebase_remote_config on Windows.

The new set() test showed Android and Apple lose the code on plain writes too, so those paths now route through the helpers each plugin already had. Windows also gets an e2e job (the suite never ran there) and the example's windows/ runner. Two Windows-only spellings are aligned with the other platforms: operation-failed to failure, write-canceled to write-cancelled. Nothing could depend on them, since no Windows code reached Dart before.

The transaction-based permission-denied test expects unknown on Windows only: the desktop SDK replaces non-datastale server errors on sent transactions with kErrorUnknownError before the plugin sees them (firebase-cpp-sdk#1904). Plain writes are unaffected.

Related Issues

• Fixes [firebase_database] Windows: a deliberate Transaction.abort() throws FirebaseException instead of returning TransactionResult(committed: false) #18549
• Fixes [firebase_database][Windows] Native RTDB error codes are lost when Pigeon/EventChannel errors omit details #18550
• Upstream, still open: Desktop RTDB: Repo::HandleTransactionResponse collapses every non-datastale server error (incl. permission_denied) to kErrorUnknownError with an empty message firebase-cpp-sdk#1904, Desktop RTDB: a transaction callback abort on a rerun completes the Future with kErrorNone firebase-cpp-sdk#1905

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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