test(stjm): close the remaining core branch coverage gap #249

Follow-up to #208 and merged #223.

The typed migration dispatch work is complete, but the repository's requirement for 100% branch coverage in core components is still not met. Aggregate coverage above 95% does not satisfy that requirement. Merging #223 did not establish a coverage-policy exception.

The last verified snapshot was aafe5a1714b01cc5e388887119a8276bff728f15, merged by #223:

MigratorReference.Typed.cs, MigratorInvokerFactory.cs, and JsonMigratableTypes reached 100% on both frameworks. UnionCaseRouting.cs reached 100% on .NET 11. The remaining uncovered branches were:

These include defensive reader checks, type-name and reflection fallbacks, and guards following earlier validation. Reachability needs to be established before deciding how to cover or simplify them.

Work

• Rebaseline coverage against current origin/main on .NET 10 and .NET 11, recording the exact commit and per-component branch counts.
• Classify each remaining uncovered core branch: reachable behavior, redundant/unreachable defensive code, or a runtime/configuration-dependent fallback.
• Add meaningful tests through supported public entry points for reachable behavior. Exercise runtime-dependent behavior with the relevant real configuration where feasible.
• For redundant branches, consider a focused, behavior-preserving cleanup with evidence that the protected behavior remains covered.
• Do not manufacture private framework state or exclude code merely to reach a percentage. Any justified exception or policy change requires explicit owner approval.

Acceptance

• Fresh coverage evidence identifies the exact commit and branch counts for each core component on both frameworks.
• Every remaining uncovered core branch has an explicit disposition supported by evidence.
• Core components reach 100% branch coverage, or the owner explicitly approves and documents a narrowly scoped exception or policy change for the remaining gap.
• The existing minimum of 95% branch coverage for other production code remains satisfied.
• Release builds are warning-free and the relevant tests pass on both frameworks.

Keep evidence in CI artifacts or the issue. Do not reintroduce the issue-208 development performance reports. Discriminator lookup changes and a general performance-documentation refresh are outside this issue's scope.