Implement JSON.SET and JSON.GET commands

[AlexJuca]

Summary

Add JSON.SET and JSON.GET commands so fkvs can store and retrieve JSON documents, similar to RedisJSON.

Motivation

fkvs has no JSON support today — the only way to store JSON is to SET the serialized string as an opaque blob, which gives no field addressing, validation, or partial updates.

Scope (minimum)

• JSON.SET <key> <path> <json> — store/replace JSON at a path
• JSON.GET <key> [path] — retrieve the document or a subtree

Decisions to make before implementation

1. Path support — root-only ($) for v1, or a subset of JSONPath ($.a.b, array indexing)? Root-only is far simpler and covers most use.
2. Storage model — store the parsed document tree (fast path access, more memory) vs store validated raw text and re-parse on path GET (simpler, slower path ops).
3. JSON parser — vendor a small C parser (e.g. cJSON / jsmn) vs hand-roll. Affects deps and the build.
4. Value type — add a new VALUE_ENTRY_TYPE_JSON to value_entry_t so GET/future TYPE can distinguish JSON from RAW/INT.
5. Protocol limit — the binary framing uses a 2-byte length prefix (core capped at 65535 bytes), so a single JSON doc must fit in one frame. Decide whether to raise this or document the limit.

Proposed v1 (minimal)

• Root-path only: JSON.SET k $ <json> validates and stores; JSON.GET k returns the doc.
• Validate-on-write: parse to confirm well-formed JSON, store as a new JSON-typed value. No field addressing yet.
• Register CMD_JSON_SET / CMD_JSON_GET, add handlers, add VALUE_ENTRY_TYPE_JSON.
• Follow-up issue for path addressing ($.a.b, array indices) once v1 lands.

Acceptance criteria

• JSON.SET k $ '{"a":1}' stores; JSON.GET k returns {"a":1}.
• Malformed JSON is rejected with an error reply.
• Integration tests cover set / get / overwrite / malformed.
• Benchmark numbers captured for comparison with the redis-stack JSON.SET results above.

Relevant code

• Command registration: src/commands/server/server_command_handlers.c (register_command, init_command_handlers)
• Command IDs: src/commands/common/command_defs.h
• Value types: src/core/hashtable.h (VALUE_ENTRY_TYPE_*)
• Protocol framing: src/networking/networking.c