feat(schema): Protobuf decode and encode for Schema Registry topics

[ritikbansod]

Implements F4 — Protobuf codecs, the last high-impact feature from the deep analysis: PROTOBUF subjects were the one Schema Registry gap — the code literally threw "Protobuf encoding arrives in phase 2b — not supported yet" while every comparable console decodes proto. Stacked on #7 (both touch pom.xml) — merge #1 → #2 → #3 → #4 → #5 → #6 → #7 → this.

How it works

• Decode (browser, live tail, /decode): after the 5-byte Confluent header, the payload's message-index prefix (1-based varints terminated by 0x00 — Confluent's documented protobuf wire shape) is stripped and the message parsed against the .proto compiled from the registry, then rendered as proto3 JSON with original field names and defaults made explicit. Defenses: nested index prefixes (>1 level) are reported as unsupported, and a payload that fails with a prefix is retried without one for non-Confluent producers; unparseable bytes come back as a reported error, never a throw.
• Encode (/encode, feeding the produce form): the JSON payload is validated against the descriptor (unknown fields rejected with the offending name) and emitted as Confluent wire bytes with the index prefix — asserted byte-for-byte in tests. Multi-message .proto files pick the message from the wire shape of the subject's declaration order, an envelope messageName, or the new optional messageName request field.
• Compilation: .proto text is parsed in-process by kafka-protobuf-provider 8.3.2 (Confluent Community License) — the same library Confluent's own deserializer uses — resolved from a new packages.confluent.io Maven repository in the pom. Compiled schemas are TTL-cached (5 min) like Avro; the cache is keyed per registry URL and schema id.

Verification

• 5 new integration tests against a mocked registry: hand-built Confluent wire fixture decodes to proto3 JSON (order_id/amount/paid preserved); encode emits 0x00 + id + 01 00 + message exactly (index prefix asserted byte-for-byte); produced wire bytes survive produce-and-browse with schema id attached; unknown JSON fields rejected; unparseable messages reported not thrown.
• Full suite: 63/63 green.

Honest limitations (documented)

• Nested-message index prefixes (subjects whose wire bytes carry multi-level indexes) are reported as unsupported rather than mis-decoded.
• Verified against the documented Confluent wire shape and a round-trip; a live Confluent Cloud/Strimzi registry round-trip is the remaining acceptance step for a release.
• kafka-protobuf-provider is Confluent Community License — a runtime dependency only; no Confluent code is copied into Kview.