[BUG][DART] fix Optional<T> fromJson wrapping

[timonrieger]

Summary

When useOptional=true (added in #22257) is enabled, the x-is-optional vendor extension correctly wraps scalar fields in Optional.present() / Optional.absent() during fromJson deserialization. However, six collection branches and the numeric branch in native_class.mustache are missing this check entirely, causing generated fromJson methods to return raw List<T>, Map<K,V>, or call the non-existent Optional<num?>.parse() constructor where Optional<T> is expected.

This produces compile errors in any Dart project using useOptional=true with optional array, map, or numeric fields.

Scalar fields (non-collection, non-numeric) already had correct x-is-optional handling at lines 290–304 of the template. This PR extends the same pattern to the remaining branches.

Relation to #23365

This PR conflicts with #23365 on two lines inside the {{^vendorExtensions.x-is-optional}} branch of the nested-array hunk (the listFromJson(json[r'...']) and .cast<T>() fixes). #23365 should be merged first, then those two conflict lines become a no-op resolution. Thus I stacked this PR on top of #23365. The two PRs are otherwise independent and fix non-overlapping issues.

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[cubic-dev-ai]

1 issue found across 15 files

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>

[timonrieger]

cc @jaumard (2018/09) @josh-burton (2019/12) @amondnet (2019/12) @sbu-WBT (2020/12) @kuhnroyal (2020/12) @agilob (2020/12) @ahmednfwela (2021/08)

[wing328]

thanks for the PR

can you please review the build failure when you've time? https://github.com/OpenAPITools/openapi-generator/actions/runs/26029167650/job/76510300895?pr=23811