[CP-beta][FGP] Report the real error when applying kotlin-android fails - #195691

…er#195080)

When `android.builtInKotlin=false` and `android.newDsl=true`, the
Flutter Gradle Plugin said "KGP was not found on the classpath". The
real error from the Kotlin Gradle Plugin was hidden. The build then
failed with an unrelated script compilation error.

This change catches only the missing-plugin case, so other failures show
the Kotlin Gradle Plugin's own message.

Related to flutter#181557 and
flutter#189236.

<details><summary>Agent authored details</summary>

`FlutterPluginUtils.detectApplyingKotlinGradlePlugin` wrapped
`pluginManager.apply("kotlin-android")` in `catch (_: Exception)`. It
now catches `org.gradle.api.plugins.UnknownPluginException`, which
Gradle's `DefaultPluginManager.apply(String)` throws for an unknown
plugin id. Failures inside the plugin's own `apply` are wrapped
differently and now propagate.

Before (AGP 9.3.1, KGP 2.4.20, template app, `builtInKotlin=false`,
`newDsl=true`):
```
Applying the Kotlin Android Plugin (KGP) was unsuccessful. KGP was not found on the classpath.
...
* What went wrong:
Script compilation errors:
  Line 41: kotlin {
```
After:
```
> Failed to apply plugin 'kotlin-android'.
   > ⛔ Failed to apply plugin 'org.jetbrains.kotlin.android'
     The 'org.jetbrains.kotlin.android' plugin is not compatible with AGP's 9.0 new DSL (`android.newDsl=true` is enabled by default).
     Solution: Set `android.builtInKotlin=true` in `gradle.properties` and migrate to built-in Kotlin (see https://kotl.in/gradle/agp-built-in-kotlin for guidance), or set `android.newDsl=false` in `gradle.properties` to temporarily bypass this issue.
```

Tests: the two existing "KGP fails to apply" tests now throw
`UnknownPluginException`. Added `propagates a KGP failure that is not a
missing plugin`.

No comments, docs or tests removed.

Verification: `./gradlew test` (JDK 17) passed. ktlint 1.5.0 with the CI
flags passed. A manual build of the template app was run before and
after.

Note: with AGP 9, KGP resolves through AGP even when
`settings.gradle.kts` does not declare it, so the missing-plugin branch
is hard to reach in practice.

This applies cleanly to `flutter-3.50-candidate.0`;
`FlutterPluginUtils.kt` is identical there.
</details>

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