[v2] Remove all [Obsolete] members

[thomhurst]

Tracking issue for the next major (v2) release: actually delete all currently [Obsolete]-marked public members.

Background

Members listed below were previously deleted in #5384 (shipped in v1.27.0), which broke semver — see #5539. They are being restored (with [Obsolete] reapplied) for the remainder of the v1.x line. The actual deletion must wait until the v2 major version bump so consumers receive proper compile-time signaling.

Members to remove in v2

TUnit.Assertions

• CountWrapper<TCollection, TItem> class — TUnit.Assertions.Conditions.Wrappers
• LengthWrapper class — TUnit.Assertions.Conditions.Wrappers
• CollectionAssertionBase<TCollection, TItem>.HasCount() — replaced by Count()
• CollectionAssertionBase<TCollection, TItem>.HasCount(int, string?) — replaced by Count().IsEqualTo(n)
• AssertionExtensions.HasLength(IAssertionSource<string>) — replaced by Length()
• AssertionExtensions.HasLength(IAssertionSource<string>, int, string?) — replaced by Length().IsEqualTo(n)

TUnit.Core

• TestBuilderContext.ObjectBag property — replaced by StateBag
• TestRegisteredContext.ObjectBag property — replaced by StateBag
• Timing record class — replaced by OpenTelemetry activity spans
• ITestOutput.Timings property
• ITestOutput.RecordTiming(Timing) method
• TestContext explicit-interface implementations of Timings / RecordTiming and the internal _timingsLock field
• Internal TimingEntry record (no longer needed once public Timing API is gone)

Process at v2 cut

1. Bump major version to 2.0.0
2. Delete the members above
3. Regenerate TUnit.PublicAPI/Tests.*_Library_Has_No_API_Changes.*.verified.txt snapshots
4. Update XML doc / migration docs to remove references to obsolete APIs
5. Close this issue and [Bug]: Removal of obsolete members is a breaking change and should cause a semver-major version bump #5539