[vm/ffi] Migrate static_checks tests from legacy multitest format

[samuelkchris]

Summary

Migrate FFI static_checks test files from legacy multitest format to modern static error test format using [cfe] and [analyzer] expectations.

Changes Made

• regress_44986_test.dart: Convert Null field type error
• regress_46085_test.dart: Convert Array annotation errors
• regress_47673_2_test.dart: Convert unknown type error
• regress_51041_test.dart: Convert nullable struct field error
• vmspecific_function_callbacks_negative_test.dart: Convert callback type errors
• vmspecific_static_checks_varargs_test.dart: Convert VarArgs errors
• vmspecific_variance_function_checks_test.dart: Convert type variance errors

Test Plan

Converted multitest markers like //# 01: compile-time error to modern static error expectations with precise error locations and messages for both analyzer and CFE.

Fixes #60212

[copybara-service]

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[copybara-service]

https://dart-review.googlesource.com/c/sdk/+/435743 has been updated with the latest commits from this pull request.

[copybara-service]

https://dart-review.googlesource.com/c/sdk/+/435743 has been updated with the latest commits from this pull request.

[kevmoo]

Looks like tests are still failing!