feat: regenerate client from corrected OpenAPI spec

[mekilis]

Summary

• Regenerates the Java client from the corrected Convoy OpenAPI spec produced by fix(docs): OpenAPI spec fidelity for generated SDK clients convoy#2742 (nullable object/map handling, bearer auth security scheme, JSON-only onboard, portal link endpoints_metadata nullability).

Test plan

• Local suite: 25/25 passing
• Wide smoke against the QA harness passed

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Note

Medium Risk
Large generated surface area and a breaking change to bulk onboard request typing; behavior shifts around null/optional JSON fields across many models.

Overview
Regenerates the Java client from the updated Convoy OpenAPI spec, with a post-generation perl step in generate.sh that fixes JsonNullable map helpers so put*Item does not NPE after setX(null) or JSON null.

Model changes are dominated by optional fields moving to JsonNullable (getters, *_JsonNullable setters, and nullable-aware equals/hashCode), plus EMPTY sentinel values on generated enums for open/null enum cases.

OnboardApi.bulkOnboard is a notable API break: the JSON body parameter is now ModelsBulkOnboardRequest (serialized as application/json) instead of a File with application/octet-stream.

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