GH-1786 - Fix NPE in FormattableType.of()

[arimu1]

Summary

Fixes #1786.

When observability renders an observed method whose parameter type involves an unbounded wildcard (for example Class<?> as used by AbstractHttpMessageConverter.canWrite/canRead, or List<?> / List<T>), ResolvableType.resolve() returns null. Feeding that into FormattableType.of(Class) NPE'd on type.getTypeName().

This is the same root cause as #1748 / #1713. Main already routes through FormattableType.of(ResolvableType), but that factory used type.toString() as a type name. That:

1. Does not correctly treat a bare ? / unresolvable type variable as the wildcard token for all call paths.
2. Embeds generic arguments in the string, so DefaultObservedModule.render then double-appended them (List<?><?>) and mangled nested generics during package abbreviation (Map<String, List<?>> → j.u.M.l.S.u.List<?>>).

Fix

FormattableType.of(ResolvableType) now:

• Asserts the argument is non-null
• Uses resolve() and maps null to the existing ? wildcard instance
• Formats only the resolved raw type so callers (e.g. DefaultObservedModule.render) can still compose generic arguments recursively

Also adds Assert.notNull on of(Class) for consistency with of(JavaClass).

Tests

• FormattableTypeUnitTests — unbounded wildcard / type variable → ?; Class<?> regression (Null Pointer in FormattableType.of() #1786)
• DefaultObservedModuleUnitTests — exact signatures for List<?>, List<T>, and Class<?>

./mvnw -pl spring-modulith-core,spring-modulith-observability/spring-modulith-observability-core -am \
  test -Dtest=FormattableTypeUnitTests,DefaultObservedModuleUnitTests \
  -Dsurefire.failIfNoSpecifiedTests=false

• FormattableTypeUnitTests: 9/9
• DefaultObservedModuleUnitTests: 4/4
• Temurin 21

Documentation

• Not needed (bug fix only)