Fix direct nullable nested updater evaluation

[AGiorgetti]

Summary

• Add a regression for direct nullable source members passed to an explicitly selected nested updater under NullableMismatch.Throw.
• Capture direct nullable updater sources with a pattern local so the source getter is evaluated once for present and null values.

Validation

• Red baseline: the new regression failed before the fix because the non-null getter was read twice.
• Focused regression: 1 passed.
• Nested-update suite: 17 passed.
• Full generator tests: 568 passed.
• Release build: 0 warnings, 0 errors.
• Release runtime tests: 5 passed.
• Release integration tests: 1 passed.
• Release packaging tests: 61 passed, 1 expected Native AOT prerequisite skip.

The solution-wide test wrapper also attempts to run sample and benchmark executables as test projects when IsTestProject=true; those non-test projects report missing testhost dependencies. They are covered by their dedicated sample and benchmark validation commands.

Closes #13