fix: honour the Materializer passed to Java DSL Route.flow

[pjfanning]

Motivation

RouteAdapter.flow(system, materializer) — the implementation behind the Java DSL Route.flow(ActorSystem, Materializer) — accepted a Materializer but only used the ActorSystem. The flow was built through the implicit RouteResult.routeToFlow conversion, and Route.toFunction hardcodes SystemMaterializer(system).materializer into the RequestContext, so the materializer supplied by Java users was silently discarded. Routes saw the system materializer in extractMaterializer, in entity(...) unmarshalling, and in any stream run from a directive.

Modification

• scaladsl/server/Route.scala: added a private[pekko] toFunction(route, materializer) overload holding the existing body; the public toFunction(route) now delegates to it with SystemMaterializer(system).materializer, so its behaviour is unchanged.
• javadsl/server/directives/RouteAdapter.scala: scalaFlow builds its handler with that overload and does the mapAsync(1) itself, so the given materializer reaches the RequestContext.

No copied code.

Result

Route.flow(system, materializer) runs the route with the materializer it was given. The flow(system) overload is unaffected — it already passed SystemMaterializer(system).materializer explicitly. Binary compatibility is preserved: the new method is private[pekko] and purely additive.

Tests

• sbt "http-tests/testOnly org.apache.pekko.http.javadsl.server.RouteFlowMaterializerTest" — 2 passed. New test asserts the materializer observed by extractMaterializer for both the explicit-materializer and system-materializer overloads. Directional: with the RouteAdapter change reverted, flowUsesTheGivenMaterializer fails with expected: <270352276> but was: <1155460560>.
• sbt "http/scalafmt" "http-tests/Test/javafmt" "http/mimaReportBinaryIssues" — clean (MiMa scoped to the changed module).
• sbt headerCreateAll — header added to the new test file.
• git diff --check — clean.

References

None - Java DSL Route.flow(system, materializer) ignored its materializer argument.