Fail fast on invalid deliverer configuration

[endrju19]

Summary

• Reject duplicate MessageDeliverer.type values at startup instead of silently replacing a deliverer (Duplicate deliverer types silently select one deliverer #117).
• Fail default processor startup when no deliverer is registered, while keeping publisher-only and custom processor configurations valid (Processor starts without a deliverer and permanently fails entries #118).
• Preserve the auto-configured OutboxEntryProcessor for applications that inject it into a custom OutboxProcessor. Also accept a custom OutboxEntryProcessor that owns its deliverer without a separate MessageDeliverer bean.

Verification

• Core and Spring auto-configuration tests on current main.
• Spring auto-configuration tests with Spring Boot 3.5.12 and 4.0.4.
• ktlintCheck for okapi-core and okapi-spring-boot.

This is independent of #116, which concerns claiming entries with unsupported delivery types across instances.