Support net10.0, remove net6.0, and upgrade packages

[gdlcf88]

Summary

• Add .NET 10 support and drop .NET 6. Multi-target projects now build for net10.0;net9.0;net8.0 (the netstandard2.1 core libraries are unchanged).
• Upgrade .NET 8/9 and library dependencies to the latest stable versions to keep the matrix consistent and error-free.

Target frameworks

net9.0;net8.0;net6.0 → net10.0;net9.0;net8.0 across all multi-target src/test projects and common.testing.props.

Dependency updates (Directory.Build.props)

Package	Old	New
EF Core (net10)	—	10.0.9 (new)
EF Core (net9)	9.0.8	9.0.17
EF Core (net8)	8.0.19	8.0.28
EF Core (net6)	6.0.36	removed
Microsoft.Extensions.*	9.0.8	10.0.9
Microsoft.NET.Test.Sdk	17.13.0	18.6.0
Grpc	2.71.0	2.80.0
Google.Protobuf	3.32.0	3.35.1
Dapper	2.1.66	2.1.79
MongoDB.Driver	3.4.3	3.9.0
Newtonsoft.Json	13.0.3	13.0.4
DistributedLock.Core	1.0.8	1.0.9

Also: package version 1.4.0 → 1.5.0, ConfigureAwait.Fody 3.4.1, coverlet.collector 6.0.4 → 10.0.1, xunit.runner.visualstudio 3.1.4 → 3.1.5.

Verified Microsoft.Extensions.* 10.0.9 still ships netstandard2.0/2.1 targets, so the netstandard2.1 core libraries remain compatible.

CI workflows

• test.yml: installed SDKs 6.0.x/7.0.x/9.0.x → 8.0.x/9.0.x/10.0.x
• publish.yml: 9.0.x → 10.0.x

Incidental fix

The C# 14 analyzer surfaced CS9336 in DtmGrpcTmClient.cs: is not null or "" parses as (is not null) or (is ""), i.e. just is not null — the or "" was dead code, contrary to the apparent intent ("not null and not empty"). Changed to is not (null or ""). Note: this changes runtime behavior for an empty-string ActionApiToken (the header is no longer added).

Testing

• dotnet build (Debug & Release): 0 errors.
• All tests pass on net10.0 and net8.0 (106 each). net9.0 compiles but wasn't run locally (runtime not installed); CI covers all three.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.64%. Comparing base (ddacc44) to head (d9f1941).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #78      +/-   ##
==========================================
- Coverage   84.81%   84.64%   -0.17%     
==========================================
  Files          99       99              
  Lines        2410     2410              
  Branches      184      184              
==========================================
- Hits         2044     2040       -4     
  Misses        312      312              
- Partials       54       58       +4     

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