[mokuzon]

Summary

Update the SDK's directly declared dependencies and migrate JSON handling.
This is the second PR in the stack and depends on #2004.

Changes

• Update declared dependencies, including OkHttp; use Gson for integration-test JJWT and remove
  unused dependency entries.
• Migrate shared mappers, JSON/YAML binding, and webhook parsing to the new Jackson APIs.
• Replace Retrofit's Jackson converter with an SDK converter, preserving request/response handling
  and when builder settings take effect.
• Preserve mapper defaults, including null primitives, timestamps, and input limits, and
  integration-test YAML behavior.
• Preserve public IOException constructors, the exception hierarchy, and IOException handling
  for JSON failures. Response I/O failures retain the original exception.
• Preserve application-defined objectMapper beans and explicitly select the SDK mapper in the
  audience-management sample to retain unknown-enum handling.

The regression tests from #2004 run against both implementations with the same assertions;
Jackson-specific imports are updated as needed.

Breaking changes

The SDK's client exception hierarchy and public IOException constructors remain compatible.
Use this table to identify changes needed in application code.

If your application…	Required change
Uses ModelObjectMapper.createNewObjectMapper(), BotAwareJacksonConverter.create(...), or LineBotAutoConfiguration.objectMapper().	Replace com.fasterxml.jackson.databind.ObjectMapper with tools.jackson.databind.ObjectMapper and recompile. The changed parameter/return types are not binary compatible.
Injects the SDK's mapper in Spring.	Update the injected type as above. If a primary jacksonJsonMapper also exists, use @Qualifier("objectMapper") to select the SDK's bean. An application-defined bean named objectMapper still takes precedence.
Changes a mapper using configure(...) or registerModule(...).	Configure a builder, such as mapper.rebuild(), and use the mapper returned by build(). Existing mappers cannot be reconfigured.
Provides custom Jackson serializers, deserializers, or modules.	Migrate them to the tools.jackson APIs. Updating only the ObjectMapper import is insufficient.
Catches Jackson-specific exceptions (com.fasterxml.jackson.*) from parseErrorBody(...), WebhookParser.handle(...), or client JSON conversion.	Handle IOException. JSON failures now use an IOException wrapper whose cause is a tools.jackson.core.JacksonException subtype. parseErrorBody(...) now declares only IOException; subclasses catching it continue to work.
Casts LineClientJsonParseException.getCause() to the old StreamReadException or DatabindException.	Treat getCause() as IOException. If Jackson-specific details are needed, the new Jackson exception is at getCause().getCause().
Catches IOException around direct mapper calls such as readValue(...) or writeValueAsString(...).	Catch tools.jackson.core.JacksonException for JSON failures; it is unchecked. Handle actual I/O failures separately where required by the method. SDK exception wrapping does not apply to direct mapper calls.
Uses classes supplied transitively by com.fasterxml.jackson.core:jackson-core, com.fasterxml.jackson.core:jackson-databind, com.fasterxml.jackson.datatype:jackson-datatype-jsr310, or com.squareup.retrofit2:converter-jackson.	Migrate those usages to the new Jackson APIs / SDK converter, or declare the dependencies explicitly. The SDK no longer supplies these artifacts.
Relies on generated record parsing to reject duplicate properties.	Validate duplicates before binding if rejection is required. Some previously rejected inputs now use the last value—for example, endpoint, then active, then another endpoint in GetWebhookEndpointResponse. There is no setting to restore the previous record-construction timing.

The shared com.fasterxml.jackson.annotation annotations remain in use. Response I/O failures still
preserve the original exception instance.