fix(firestore,windows): report unimplemented for sum/average aggregations

[m1roxx]

Description

On Windows, sum() and average() aggregations are silently unsupported.

The Firebase C++ SDK exposes only Query::Count() — there is no sum or average aggregation in firebase/firestore/aggregate_query.h. The Windows plugin handled that by printing to stdout and moving on:

case AggregateType::kSum:
  std::cout << "Sum is not supported on C++" << std::endl;
  break;

Two user-visible consequences:

• collection.aggregate(count(), sum('foo')) resolves successfully with the count filled in and getSum('foo') returning null. getSum is typed double?, so this is indistinguishable from a query that legitimately has no sum — the app silently reads wrong data.
• collection.aggregate(sum('foo')) leaves aggregate_query default constructed, so AggregateQuery::Get() returns FailedFuture (aggregate_query.cc) and the caller gets an opaque error with no indication that the operation is unsupported on this platform.

This PR rejects any aggregation request containing sum() or average() with an explicit unimplemented error, following the precedent set for Windows Storage listing in #18449 ("throw unimplemented instead of returning misleading empty results"). count() is unaffected.

While adding the integration test I found that MethodChannelAggregateQuery.get is the only Firestore method-channel call not wrapped in convertPlatformException, so the error reached callers as a raw PlatformException instead of a FirebaseException. That is fixed here too, the same way MethodChannelQuery.get does it — it also affects Android/iOS, where any aggregate-query failure (for example permission-denied) currently surfaces in the wrong exception type.

Related Issues

No existing issue — found while auditing the Windows plugin. Happy to file one if you'd prefer the report tracked separately.

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.

An app that today reads null from getSum() on Windows will now see a FirebaseException. That is the point of the change: the previous value was wrong, not absent.

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