chore(deps): pin FluentAssertions to 6.12.0 to keep packages MIT-licensed

[begeistert]

Description

FluentAssertions 7.x and later moved to a commercial
Xceed license; 6.12.0 is the last
MIT-licensed release. The published RP2040Sharp.TestKit package took a transitive
dependency on FluentAssertions 8.8.0, which would impose non-MIT terms on anyone
consuming the TestKit. This PR pins FluentAssertions back to 6.12.0 so the whole
dependency graph stays MIT-compatible.

Changes

• Pin FluentAssertions to 6.12.0 in RP2040.TestKit, RP2040Sharp.Tests, and
  RP2040Sharp.IntegrationTests.
• Port the TestKit's custom assertions (CortexM0Assertions, GpioAssertions,
  UartProbeAssertions) back to the FluentAssertions 6.x extension model — constructor
  without AssertionChain, using Execute.Assertion instead of the 8.x chain object.
• Add the 1.0.0-rc.2 entry to CHANGELOG.md.

No functional or API changes to the emulator itself. The packed
RP2040Sharp.TestKit nuspec now declares FluentAssertions 6.12.0.

Tests: 445 unit + 149 integration — all green.

Type of change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 🧹 Refactoring (no functional changes, just code cleanup)

💥 The public RP2040Sharp.TestKit package drops its FluentAssertions dependency
from a major 8.x → 6.x. Consumers that rely on FluentAssertions 7.x/8.x APIs
would be affected. Caught before a stable release (RC stage).

Checklist

• 🧪 Tests passed locally (dotnet test)
• ✅ Added new tests for this feature/fix
• 📝 Documentation updated (if applicable)
• 🚫 No "Magic Strings" or hardcoded values
• 💾 Validated against the RP2040 Datasheet (if applicable)

Screenshots / Hex Dumps (Optional)