[kafka] Handle basic Produce requests independently of storage - #4284

Catch failures while mapping asynchronous handler results so the dispatcher
future completes exceptionally instead of remaining pending. Cover handler
futures that fail before and after dispatch registers its callback.

Validated with Java 11: Maven reactor build and 17 targeted Kafka tests,
including Checkstyle, Spotless, and license checks.

Co-Authored-By: Codex <noreply@openai.com>
AI-Model: gpt-6
AI-Contributed/Feature: 27/27
AI-Contributed/UT: 28/28

Route requests through the dispatcher and advertise only implemented APIs.
Return version-aware errors for unsupported APIs and invalid requests.

Validated with mvn -o -pl fluss-kafka verify (23 unit tests and 1 IT).

Co-Authored-By: Codex <noreply@openai.com>
AI-Model: gpt-6
AI-Contributed/Feature: 313/313
AI-Contributed/UT: 137/137

Extract topic identity and the raw/string table mapping contract before Metadata. Validate table kinds, field projections and metadata columns independently of request handling and record decoding.

Validation: Java 11, mvn -o -pl fluss-kafka clean verify (47 unit tests and 2 integration tests); Checkstyle, Spotless and RAT passed.

Co-Authored-By: Codex <noreply@openai.com>
AI-Model: gpt-6
AI-Contributed/Feature: 769/769
AI-Contributed/UT: 480/480

Resolve database.table topic names directly to Fluss table paths and return
qualified names for metadata. Validate name boundaries and cover same-named
tables in different databases through native metadata round trips.

Read projected field names directly from DataField to avoid rebuilding the
complete field-name list for every projected column.

Validated on Java 11: 67 Kafka unit tests and 2 integration tests passed,
along with Checkstyle, Spotless, RAT, and git diff --check.

Co-Authored-By: Codex <noreply@openai.com>
AI-Model: gpt-6
AI-Contributed/Feature: 52/52
AI-Contributed/UT: 167/167

Integrate the DDL mapping prerequisite and use its resolver for Metadata discovery. Omit unsupported tables from all-topic queries and return per-topic mapping errors for named queries. Preserve metadata for compatible tables in mixed requests.

Validation: Java 11, mvn -o -pl fluss-kafka clean verify (62 unit tests and 2 integration tests); Checkstyle, Spotless and RAT passed.

Co-Authored-By: Codex <noreply@openai.com>
AI-Model: gpt-6
AI-Contributed/Feature: 742/742
AI-Contributed/UT: 571/571

Route Metadata queries through the database.table mapping from PR03 and
enumerate compatible topics across databases. Preserve brokers and valid
topics when a requested database is absent or disappears during discovery;
propagate other gateway failures.

Cover qualified names, same-named tables across databases, synchronous and
asynchronous lookup failures, and Metadata v0-v11 over a real listener.

Validated on Java 11: 90 unit tests and 4 integration tests passed, along
with Checkstyle, Spotless, RAT, and git diff --check. Five additional
interleaved asynchronous review probes passed.

Co-Authored-By: Codex <noreply@openai.com>
AI-Model: gpt-6
AI-Contributed/Feature: 129/129
AI-Contributed/UT: 514/514

Validate non-idempotent Produce v3-v11 and isolate invalid partitions before invoking a protocol-independent backend. Preserve partition order, acknowledgements and owned record data. Leave production registration to the append integration PR.

Validation: Java 11, mvn -o -pl fluss-kafka spotless:apply clean verify; 69 unit tests and 2 integration tests passed, including Checkstyle, Spotless and RAT.

Co-Authored-By: Codex <noreply@openai.com>
AI-Model: gpt-6
AI-Contributed/Feature: 621/621
AI-Contributed/UT: 366/366

Read compressed records through a closeable streaming iterator to avoid
preallocating a record list from the declared batch count. Reject incoming
Kafka batches whose offset range disagrees with their record count before
copying records or invoking the backend. Keep Fluss-assigned offsets independent
of this input validation and preserve partition error isolation.

Cover all five codecs, owned record copies, malformed counts and offset ranges,
and backend-assigned offsets. Java 11 validation passed: 117 unit tests and
4 integration tests, plus Checkstyle, Spotless, RAT, and git diff --check.

Co-Authored-By: Codex <noreply@openai.com>
AI-Model: gpt-6
AI-Contributed/Feature: 42/42
AI-Contributed/UT: 168/168