Harden interval tree API tests

[Courtland9777]

Summary

• Add focused edge-case coverage for point queries, overlap queries, duplicates, empty intervals, no-match cases, nested intervals, adjacent half-open intervals, DateTime intervals, and deeper removal behavior.
• Add deterministic randomized add/remove/query tests checked against a simple in-test brute-force list model.
• Keep the public API and implementation unchanged because no correctness bug was exposed.

Validation

• git diff --check
• dotnet format IntervalTree.Net.sln --verify-no-changes
• dotnet restore IntervalTree.Net.sln
• dotnet build IntervalTree.Net.sln --configuration Release --no-restore
• dotnet test IntervalTree.Net.sln --configuration Release --no-build
• dotnet pack src/IntervalTree.Net/IntervalTree.Net.csproj --configuration Release --no-build

All validation passed using temporary NuGet cache paths under /tmp to avoid the known local cache permission issue. No publication was performed.