Update dependencies and lang version

[nmummau]

I started out making changes to the solution dependencies. I then noticed that there were some other stylistic/modernizations that could be made. I isolated the commits in this PR and detailed each commit. I also converted namespaces to be file-scoped which is the reason for the heavy diff.

In summary:

• Updated deps
• Updated lang version to 12
• Convert namespace to file-scoped

In Detail

chore(deps): bump Microsoft.SqlServer.DacFx from 170.1.61 to 170.2.70

Updated the DacFx package reference to the latest patch version to incorporate upstream fixes and improvements.

• Microsoft.SqlServer.DacFx: 170.1.61 → 170.2.70
• No other dependencies or build settings changed

build(tests): migrate to MSTest v4 and adopt Assert.ThrowsExactly; update test SDK

• Update dev test deps:
  • Microsoft.NET.Test.Sdk: 17.14.1 → 18.0.0
  • MSTest.TestAdapter: 3.9.3 → 4.0.1
  • MSTest.TestFramework: 3.9.3 → 4.0.1
• Replace Assert.ThrowsException → Assert.ThrowsExactly where exact type is required
• Remove unused using Microsoft.SqlServer.Management.HadrModel;

Rationale: MSTest v4 removes deprecated ThrowsException APIs; tests updated in lockstep with package upgrades to keep the build passing.

build(tests): enable nullable refs and bump LangVersion to C# 12 in test project

• Set to 12 (was 9.0)
• Enable nullable reference types: enable
• No changes to target frameworks, packages, or test settings

Aims to modernize the test project to current C# features and improve null-safety.

style: cleanup code using analyzers and fix spelling error

• Applied Visual Studio analyzer quick actions (Alt+Enter) to remove unused assignments and simplify code
• Fixed spelling mistake: seperated → separated
• No functional or behavioral changes

style: modernize project and source formatting

• Bump C# language version from 9.0 → 12 in project file
• Convert namespace to file-scoped
• Apply consistent code style using analyzers
• No functional or behavioral changes

[ErikEJ]

@nmummau Could you have a look at the failing tests?

[ErikEJ]

@nmummau Also, I my opinion, multiple smaller focused PRs is preferable

[nmummau]

@nmummau Also, I my opinion, multiple smaller focused PRs is preferable

I agree and understand. I'll abandon this PR and start a new one(s)