Fix nullable OpenAPI 3.1 container types for typescript-axios - #23778

Fixes #19890

This updates nullable detection for OpenAPI 3.1 schemas whose types include null, then propagates that into generated CodegenProperty values. The typescript-axios model template already emits | null when isNullable is set, so nullable array/object/map properties now generate the expected union types.

cc @nicokoenig for typescript-axios

Tests:

• JAVA_HOME="$(brew --prefix openjdk@17)" PATH="$(brew --prefix openjdk@17)/bin:$PATH" ./mvnw -B -ntp -pl modules/openapi-generator -am -Dtest=org.openapitools.codegen.typescript.axios.TypeScriptAxiosClientCodegenTest#generatesNullUnionsForOpenApi31NullableContainers -Dsurefire.failIfNoSpecifiedTests=false test
• git diff --check

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work.
• Run the full project build and sample regeneration commands from the template. Not run locally due disk constraints; this PR adds a focused regression test and does not change templates or checked-in samples.
• File the PR against master.
• Reference the reported issue using GitHub linking syntax.
• Mention the relevant language/generator maintainer.

Summary by cubic

Fix nullable detection for OpenAPI 3.1 container schemas so the typescript-axios generator emits | null unions for object, array, and map properties. Fixes #19890.

• Bug Fixes
  • Treat type: ['object'|'array', 'null'] as nullable via ModelUtils.isNullable, including refs and top-level.
  • Propagate isNullable to CodegenProperty so existing templates output | null.
  • Add a targeted test and 3.1 fixture verifying unions for validation, requirements, and settings.

^{Written for commit df1d446. Summary will update on new commits.}