rust: fix rust generation of optional additionalProperties in openapi 3.1.0

[tommilligan]

Closes #16119

cc Rust technical committee: @frol @farcaller @richardwhiuk @paladinzh @jacob-pro

Fixes code generation for rust client models, when the generated type of the subfield is optional with additionalProperties, in openapi 3.1.0

This PR inserts a default value None for the generated type Option<Option<HashMap<K, V>>> to fix what is currently a compilation error (please see #16119 for details, and reproducing the original issue).

The root cause of the above bug seems to be that in the mustache template, Option<T> seems to be considered sometimes isContainer: true and sometimes isContainer: false depending on the value of T, leading to unexpected behaviour.

This PR also contains a regression test for the issue, and adds this regression test to the relevant CI job. Please advise if this should be located/structured differently - I see modules/openapi-generator/src/test/resources/bugs already exists, but could not see it referenced in CI anywhere.

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[wing328]

i think it should be others instead of other. Same for below line 11.

[tommilligan]

Ah yes excellent spot. Corrected

[wing328]

FYI. I've filed #17624 to fix the map, free form object detection.

there's a slight change in the rust sample output and it compiles without issue. (I've update the model template in rust client generator to properly handle free form object)