refactor(testcontainers): extract ObjectMapper to static field in tdg.Utils

[dkasimovskiy]

Hoist the ObjectMapper instance used by the test helper
org.testcontainers.containers.tdg.Utils from a method-local variable to
a static final field.

ObjectMapper instances are thread-safe once configured, so the previous
new ObjectMapper() allocations in sendUsers(List<User>, TDGContainer<?>)
and getUsers(int, TDGContainer<?>) were unnecessary and duplicated.
Extracting the instance:

• removes the per-call allocation and the duplicated new ObjectMapper()
  call sites;
• keeps the mapper in a single, clearly named place (OBJECT_MAPPER),
  matching the UPPER_SNAKE_CASE convention for static final fields;
• makes future configuration of the mapper (modules, features) a
  one-line change in a single spot instead of two.

This is a test-only refactor — Utils lives under src/test/java and is
not part of the published artifact, so there is no API or behavior change
for SDK consumers.

I haven't forgotten about:

• Tests
• Changelog
• Documentation
  • JavaDoc was written
• Commit messages comply with the guideline
• Cleanup the code for review. See checklist