Move suspend/reactive route validation to processor

[yawkat]

Summary

• add a compile-time route validation rule for suspended controller methods that declare reactive or async return types
• make RouteValidationVisitor recognize Kotlin/KSP route methods where HttpMethodMapping is exposed as a stereotype rather than a direct annotation
• remove the runtime guard from DefaultRouteInfo
• extend the Kotlin test harness so tests can opt into additional KSP symbol processor providers
• add a focused end-to-end Kotlin compile test that exercises RouteValidationVisitor through a narrow route-only visitor provider

Why

Issue #11131 reproduces a bad failure mode when a suspended controller method returns a reactive type after prior suspension. Rather than rejecting that shape during route construction at runtime, this change moves the unsupported-signature check into the existing http-validation processor path so developers get earlier feedback.

Implementation details

• SuspendedReactiveReturnTypeRule now checks the declared suspended method return type and explicitly rejects kotlinx.coroutines.flow.Flow in addition to other async/reactive types
• RouteValidationVisitor now falls back to stereotype-aware lookup for HttpMethodMapping, which is necessary for Kotlin/KSP route methods annotated with @Get
• inject-kotlin-test now supports opt-in extra KSP SymbolProcessorProviders without changing default behavior for existing tests
• the end-to-end test uses a narrow route-only provider built on TypeElementSymbolProcessor, avoiding unrelated service-loaded visitors from polluting the test path

Verification

Targeted end-to-end verification passes with:

./gradlew :micronaut-inject-kotlin:test --tests io.micronaut.kotlin.processing.visitor.SuspendedReactiveRouteValidationSpec \
  :micronaut-inject-kotlin-test:compileGroovy \
  :micronaut-http-validation:compileJava \
  :micronaut-router:compileJava

Results:

• suspend fun hello(): String compiles
• suspend fun hello(): Flow<String> fails during KSP/visitor processing with:
  • Unsupported suspended controller return type [kotlinx.coroutines.flow.Flow]. Suspend functions must not return reactive or async types.

Notes

• this PR intentionally removes the earlier runtime guard in favor of processor-based validation
• the Kotlin test harness change is additive: existing helper behavior remains unchanged unless a test explicitly opts into extra symbol processor providers

Fixes #11131

[sonarqubecloud]

Quality Gate passed

Issues
3 New issues
0 Accepted issues

Measures
0 Security Hotspots
92.3% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud