Fix SQLServer 2016 tag table compatibility bug

[Arkatufus]

Fix TagTable mode on SQL Server < 2017

Problem

When using tag-write-mode = TagTable with SQL Server 2016 (or any version prior to 2017), the read journal throws:

LinqToDB.Linq.LinqException: '...StringAggregate(";", r => r.TagValue).ToValue()' cannot be converted to SQL.

This happens because LinqToDB's StringAggregate maps to SQL Server's STRING_AGG function, which was introduced in SQL Server 2017. Users on SQL Server 2016 or earlier cannot use TagTable mode at all.

Solution

Added runtime SQL Server version detection via LinqToDB's SqlServerDataProvider.Version property. When the detected version is below 2017, tag aggregation falls back to a client-side implementation that:

1. Fetches journal rows first
2. Queries tag rows separately using WHERE OrderingId IN (...)
3. Joins tags to journal rows in memory

This is transparent to the user -- no configuration changes needed. The generic provider-name = "SqlServer" works correctly because LinqToDB auto-detects the version by querying sys.databases for the database compatibility level.

Changes

Core fix (2 files):

• AkkaDataConnection.cs -- Added SupportsStringAggregate property that checks SqlServerDataProvider.Version >= v2017 at runtime. Returns true for all non-SQL Server providers (PostgreSQL, MySQL, SQLite all support equivalent functions).
• BaseByteReadArrayJournalDao.cs -- Split AddTagDataFromTagTableAsync into two paths: AddTagDataFromTagTableWithStringAggregateAsync (existing SQL path) and AddTagDataFromTagTableClientSideAsync (new fallback). The dispatcher checks connection.SupportsStringAggregate to choose.

Test infrastructure (2 files):

• SqlServer2016Container.cs -- Runs SQL Server 2022 but sets COMPATIBILITY_LEVEL = 130 to simulate SQL Server 2016. Uses generic "SqlServer" provider name for auto-detection.
• SqlServer2016PersistenceSpec.cs -- xUnit collection definition for the 2016 compat fixture.

Tests (9 files):

• SupportsStringAggregateSpec.cs -- Unit tests verifying version detection returns correct values for v2005-v2016 (false), v2017+ (true), and non-SQL Server providers (true). No database required.
• SqlServer2016EventsByTagSpec.cs -- Includes a proof test that directly calls StringAggregate and asserts it throws on compat level 130, plus the standard TCK EventsByTag tests running through the fallback path.
• SqlServer2016CurrentEventsByTagSpec.cs, SqlServer2016AllEventsSpec.cs, SqlServer2016CurrentAllEventsSpec.cs, SqlServer2016EventsByPersistenceIdSpec.cs, SqlServer2016PersistenceIdsSpec.cs, SqlServer2016QueryThrottleSpecs.cs -- Full TagTable integration test suite running against the 2016 compat container.

[Aaronontheweb]

LGTM

[Aaronontheweb]

LGTM

[Aaronontheweb]

LGTM