Fix views-react feature failing in native CLI due to classpath directory lookup

[lissam1]

Summary

Fixes a NullPointerException when using the views-react feature with the native mn CLI. The issue was caused by relying on classpath directory lookup, which does not work reliably in GraalVM native images.

Problem

Running:

mn create-app --features=views-react demo1

resulted in:

| Error null
java.lang.NullPointerException
    at io.micronaut.starter.feature.view.React.apply(...)

This occurred in:

• installed mn CLI
• native image build

Root Cause

The implementation used:

getResource("views/react")

to resolve a directory and then built child URLs from it.

While this works on the JVM, in GraalVM native image:

• individual resources are included
• but directory resources are not accessible via getResource

As a result:

getResource("views/react") == null

leading to the NPE.

---

Solution

Replace directory-based lookup with per-file resource resolution:

var resourceUrl = Objects.requireNonNull(
    classLoader.getResource("views/react/" + fileName),
    () -> "Missing bundled resource " + resourceName
);

This ensures:

• compatibility with native image
• explicit failure if a resource is missing
• no reliance on directory classpath behavior

---

Changes

• Removed directory-based resource resolution
• Switched to per-file lookup for all frontend templates
• Added clear error messages using Objects.requireNonNull
• Removed unused URL / MalformedURLException logic

---

Validation

Tested successfully with:

./gradlew clean :starter-core:build 
./gradlew micronaut-cli:nativeCompile --no-daemon
./starter-cli/build/native/nativeCompile/mn create-app --stacktrace --features=views-react demo1nativecheck

Closes: #2709