[kafka] Implement Metadata API for pre-created Fluss tables

[gyang94]

Parent issue

Part of #4185 — Kafka Protocols Compatibility: Basic Produce.

Motivation

Standard Kafka producers need metadata discovery before they can send records. Expose tables created through Fluss DDL as Kafka topics while preserving the DDL-defined table and schema lifecycle.

Scope

• Implement Kafka Metadata v0-v11 over a narrow metadata backend and the TabletServer gateway.
• Reuse the table/topic mapping and schema validation contract from [kafka] Define DDL table mapping for Kafka compatibility #4275; return brokers, partitions, leaders, replica assignments, ISR, and offline replicas for compatible tables.
• Preserve the Kafka listener context when resolving advertised endpoints.
• Handle invalid or missing topics, unavailable leaders, gateway failures, and table-deletion races.
• Respect authoritative ISR when bucket metadata provides it, with a conservative available-leader fallback for legacy metadata.
• Register Metadata so ApiVersions advertises the implemented capability.

Acceptance criteria

• Named-topic, all-topic, and empty-topic discovery follow the DDL mapping contract and return the expected broker and partition metadata.
• Unsupported table mappings do not affect valid topics in the same request.
• Version boundaries and invalid requests, including missing names in v10/v11, are handled correctly.
• Replica/ISR reporting, legacy fallback, and error paths have regression coverage.
• Relevant handler, mapping, transport, and integration tests and style checks pass.
• The focused implementation is reviewed and merged after its prerequisites.

Dependencies

PR04 in the foundation sequence. Depends on the PR03 DDL table mapping contract (#4275), which builds on #4265 and #4264.

Out of scope

Kafka CreateTopics/DeleteTopics, automatic table creation, Metadata v12+ or ID-only topic lookup, Produce, authentication, and JSON-to-row conversion. Schema-aware JSON Produce is a later issue under #4185; this stage only provides discovery for pre-created tables.

Implementation and review

Implementation PR: #4261.

The PR remains a draft while its prerequisites are pending. Its description includes an incremental comparison for reviewing Metadata alone.