perf(json): cache JsonMapper instance in jsonMapper()

[vincerevu]

Summary

This PR optimizes jsonMapper() by returning a single lazily initialized and cached JsonMapper instance instead of building a new mapper on every call.

What changed

• replaced per-call JsonMapper.builder()...build() construction with a cached singleton-style mapper
• kept mapper configuration unchanged while ensuring initialization happens only once

Why

Building a Jackson mapper is relatively expensive because it registers modules and applies configuration during construction. Since jsonMapper() can be called frequently across parsing and serialization paths, recreating the mapper on every invocation adds unnecessary CPU and allocation overhead.

Impact

• reduces repeated mapper construction cost
• lowers CPU and memory allocation overhead for repeated jsonMapper() calls
• preserves behavior while improving efficiency

Notes

Jackson ObjectMapper / JsonMapper instances are thread-safe after configuration, so reusing a single initialized instance is safe here.

Verification

• reviewed the refactor to confirm mapper configuration is preserved
• confirmed initialization now occurs only once
• expected performance gains are concentrated in code paths that call jsonMapper() repeatedly