Spanner Outbox/Inbox emulator tests collide on concurrent DDL (need serialized collection)

## Summary

After #4162 (missing `EmulatorDetection`) is fixed, ~28 Spanner Outbox/Inbox tests fail against the emulator with:

```
Google.Cloud.Spanner.Data.SpannerException : Operation was rejected because the system is not in a state required for the operation's execution.
---- Grpc.Core.RpcException : Status(StatusCode="FailedPrecondition", Detail="Schema change operation rejected because a concurrent schema change operation or read-write transaction is already in progress.")
```

The Spanner **emulator serialises DDL**, but the Outbox (`SpannerBinary`/`SpannerText` × `Sync`/`Async`) and Inbox test classes run in parallel, and their fixture `CREATE`/`DROP TABLE` calls collide on the shared `brightertests` database. The passing `BoxProvisioning` tests avoid this via `[Collection("SpannerBoxProvisioning")]`; the Outbox/Inbox test classes have no such collection.

This was **previously masked** by the ADC fixture-init failure from #4162 — once `EmulatorDetection` lets the tests reach the DDL stage, the concurrency limitation surfaces. Different root cause (test parallelism vs emulator DDL serialisation).

## Evidence

- Each affected test **passes in isolation**.
- Full `~Spanner` filter with collection parallelisation **disabled** is green:
```bash
SPANNER_EMULATOR_HOST=localhost:9010 GOOGLE_CLOUD_PROJECT=brighter-tests \
dotnet test tests/Paramore.Brighter.Gcp.Tests/Paramore.Brighter.Gcp.Tests.csproj -f net9.0 \
--filter "FullyQualifiedName~Spanner" -- xUnit.ParallelizeTestCollections=false
# 94 passed / 0 failed
```
Under the default parallel run: ~66 passed / ~28 failed (which tests fail is nondeterministic — it's a race).

## Candidate fix (test-only)

Serialise the Spanner Outbox/Inbox test classes:
- Place them in a shared xUnit collection (mirroring `BoxProvisioning`'s `[Collection("SpannerBoxProvisioning")]`), **or**
- Disable parallelisation for that set.

## Context

Surfaced during the `/bugfix` workflow for #4162. The #4162 fix (production `SpannerConnectionProvider`/`SpannerUnitOfWork` + test `Const` now opt into `EmulatorDetection.EmulatorOrProduction`) is complete and green; this concurrency issue is tracked separately here.

Start with the Spanner Outbox and Inbox test classes, comparing their setup with the BoxProvisioning tests and their SpannerBoxProvisioning collection. Run the shown dotnet test command with collection parallelisation enabled to reproduce the race, then apply a shared xUnit collection or equivalent serialization and rerun the Spanner filter; the tests should pass concurrently.

Written by the indexing model from the issue text.