fix: migrate engine no-op SPI defaults from @ApplicationScoped to @DefaultBean

[mdproctor]

Problem

Bare @ApplicationScoped no-op SPI defaults in the engine runtime collide with consumer implementations when the engine is indexed alongside a consumer repo's @QuarkusTest classpath. CDI sees two qualifying beans and fails with an ambiguity error. The reactive variants use @Alternative, which avoids collision but requires explicit activation per consumer.

Fix

Annotate all nine no-op/empty defaults with @DefaultBean @ApplicationScoped. @DefaultBean yields automatically to any non-default qualifying bean — the correct semantic for a platform fallback.

Blocking variants — runtime/src/main/java/io/casehub/engine/internal/worker/:

• NoOpWorkerProvisioner — bare @ApplicationScoped → @DefaultBean @ApplicationScoped
• NoOpCaseChannelProvider — bare @ApplicationScoped → @DefaultBean @ApplicationScoped
• NoOpWorkerStatusListener — bare @ApplicationScoped → @DefaultBean @ApplicationScoped
• EmptyWorkerContextProvider — bare @ApplicationScoped → @DefaultBean @ApplicationScoped

Diff variant — runtime/src/main/java/io/casehub/engine/internal/diff/:

• NoOpContextDiffStrategy — @Alternative → @DefaultBean

Reactive variants (migrate from @Alternative to @DefaultBean):

• NoOpReactiveWorkerProvisioner
• NoOpReactiveCaseChannelProvider
• NoOpReactiveWorkerStatusListener
• EmptyReactiveWorkerContextProvider

Acceptance

• All nine beans annotated @DefaultBean @ApplicationScoped
• mvn test -pl runtime passes with no regressions
• Protocol PP-20260514-engine-spi-noops-defaultbean updated to include all nine beans

Protocol

PP-20260514-engine-spi-noops-defaultbean — Engine SPI no-op defaults must use @DefaultBean

[mdproctor]

All nine beans migrated to @DefaultBean @ApplicationScoped (io.quarkus.arc.DefaultBean). Protocol updated. 506 tests pass. Follow-up: engine#258 (JsonPatchContextDiffStrategy coherence).