Remove FluentAssertions and standardize on AwesomeAssertions

[cwinland]

Current repo state already leans toward AwesomeAssertions, but the migration is incomplete:

• Directory.Packages.props still carries both AwesomeAssertions and FluentAssertions
• FastMoq.Tests/FastMoq.Tests.csproj still references FluentAssertions
• shared test global usings still expose FluentAssertions in FastMoq.Tests/GlobalUsings.cs and FastMoq.Tests.Web/GlobalUsings.cs
• docs and examples already mostly use AwesomeAssertions

This issue tracks fully removing FluentAssertions from the live repo surface and standardizing on AwesomeAssertions instead.

Refined plan

• inventory all non-generated FluentAssertions references in project files, shared usings, test code, docs, and examples
• replace remaining FluentAssertions package references and namespace imports with AwesomeAssertions
• remove the central FluentAssertions package version once no live project depends on it
• validate that no non-generated FluentAssertions references remain after the migration
• keep the work scoped to assertion-library replacement and avoid mixing in unrelated test refactors

Acceptance checklist

• All non-generated FluentAssertions package references in the repo are replaced with AwesomeAssertions or removed.
• Shared usings and direct namespace imports are updated so live test projects use AwesomeAssertions consistently.
• Docs, examples, and sample projects do not send mixed signals about which assertion library FastMoq uses by default.
• Directory.Packages.props no longer includes FluentAssertions.
• Validation confirms no remaining non-generated repo references to FluentAssertions.

Non-goals

• Do not bundle unrelated test cleanup or assertion-style rewrites beyond what is needed for the library migration.
• Do not treat generated obj or restore artifacts as migration targets.