Jumoo.Json: dependency updates and v17 → v18 forward merge

Repo: Jumoo/Jumoo.Json

Merged dependabot PRs on v17/main: codeql-action 4.37.8 → 4.37.9, Microsoft.NET.Test.Sdk 18.4.0 → 18.10.1
Dependabot closed the coverlet.collector 10.1.0 and xunit.runner.visualstudio 4.0.0 PRs as "no longer updatable" after rebasing; bumped both by hand in #26
Deleted the stale main branch; repo is down to v17/main and v18/main
First forward merge of v17/main into v18/main since the lines split at release/17.0.1 (#27, merge commit rather than squash so the next run has a baseline): carried the Test.Sdk, coverlet and codeql-action bumps; everything else on v17 was already on v18
Corrected CLAUDE.md: the repo's default branch is v17/main, not v18/main