[fix][broker] Support schema-less auto-produce registration - #27111

Motivation

Fixes #27048

Modifications

The protocol now identifies AUTO_PRODUCE_BYTES producers and advertises whether a broker accepts their schema-less registration. A supporting broker accepts that registration and enforces that each produced message carries a non-empty schema version when schema validation is enabled. This avoids unintended topic-schema registration while preserving schema-validation guarantees.

• Add the producer marker and the broker feature flag to the wire protocol.
• Omit the producer schema only when the connected broker advertises support; retain the existing registration behavior with older brokers.
• Register message schemas on demand for producers registered without a schema, and cover global/topic validation, batching, deletion, protocol, and legacy-broker behavior with tests.
• Propagate the capability through direct Proxy connections and conservatively disable it for lookup Proxy connections without a known backend.

Does this pull request potentially affect one of the following parts

No.

Compatibility: new clients retain the existing schema-carrying registration when connected to older brokers, while older clients ignore the new optional protocol fields when connected to newer brokers. A new Proxy forwards the capability only when its directly connected backend broker advertises it; lookup Proxy connections conservatively use the legacy behavior.