Make GenerateJavaTask.schemaPaths lazy instead of @Input List<Any> (fixes #952)

[harshit6392]

Fixes #952

What changed

GenerateJavaTask.schemaPaths was a plain var schemaPaths = mutableListOf<Any>(...) annotated
@InputFiles. This PR changes it to a proper ConfigurableFileCollection, mirroring how
dgsCodegenClasspath is already declared in the same class:

• schemaPaths is now a val schemaPaths: ConfigurableFileCollection, annotated with
  @get:InputFiles and @get:PathSensitive(PathSensitivity.RELATIVE).
• Three setSchemaPaths overloads are added — accepting Iterable<Any>, Provider<out Iterable<Any>>,
  and FileCollection — all delegating to schemaPaths.setFrom(...).
• generate() now resolves schemaPaths.files lazily at task-execution time instead of eagerly
  mapping the old List<Any> via Paths.get(it.toString()).

Why

• Laziness: the old field was a plain configuration-time value, so any input coming from a
  Provider (e.g. a schema location resolved from an external dependency/configuration) had to be
  resolved at configuration time — exactly what Gradle's lazy configuration model is meant to avoid.
• Correct up-to-date checking: List<Any> of raw strings doesn't give Gradle proper
  content-based snapshotting. ConfigurableFileCollection + @PathSensitive(RELATIVE) brings
  schemaPaths in line with how dgsCodegenClasspath is already handled, so Gradle can correctly
  determine task avoidance.
• Type safety: with List<Any>, passing something like a Set<File> as a single list element
  silently stringified into [/path/to/schema.graphqls] (brackets included), only surfacing as a
  downstream compilation failure. A FileCollection-backed property can't be misused this way.

Backward compatibility

Existing build scripts using schemaPaths = ["${projectDir}/src/main/resources/schema"] continue to
work unchanged — in Groovy, schemaPaths = [...] resolves to a call to setSchemaPaths(Iterable<Any>),
one of the added overloads. No existing build.gradle files in this repo's test fixtures needed
changes.

In addition to the existing string-list form, schemaPaths can now also be set to a FileCollection
or a Provider<Iterable<Any>>, e.g.:

schemaPaths.setFrom(someExternalConfiguration.map { it.files })

Testing

• Existing Gradle functional tests (CodegenGradlePluginTest, CodegenGradlePluginSpringBootSmokeTest,
  CodegenGradlePluginEntitySmokeTest) continue to exercise the string-list schemaPaths = [...] form
  used across all test-project fixtures.