fix: normalize OAS 3.1 schemas with type:[object,"null"] to set nullable:true correctly

[Picazsoo]

In OAS 3.1, nullability is expressed as a type array, e.g. type: [object, "null"]. The existing NORMALIZE_31SPEC normalization path failed to handle schemas that used this syntax and also had properties - it would not set nullable: true.

The downstream effect: a $ref to such a schema in Kotlin-Spring would incorrectly receive @field:JsonSetter(nulls = Nulls.FAIL), which is wrong for nullable fields. This is how this bug was identified in #24139. Hence this fixes #24139.

I had pretty big issues with failing CircleCI on node2. I discussed with github copilot how to fix the issues and the current approach using setup-cpp was flagged as outdated and unnecessary. I implemented the suggested fix and it fixed the issues right away. I am tagging the C++ comittee to take a look at these unrelated changes as my experience with C/C++ is extremely limited - @ravinikam, @stkrwork, @etherealjoy, @MartinDelille, @muttleyxd, @aminya

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Summary by cubic

Sets nullable:true for OAS 3.1 schemas whose type arrays include "null" while preserving properties, and makes Kotlin Spring skip @JsonSetter(nulls = Nulls.FAIL) on nullable $ref.
Stabilizes C++ CI by installing build tools via apt, adding "." to cmake, forcing clang via CMAKE_C_COMPILER/CMAKE_CXX_COMPILER, and removing unnecessary ninja-build.

^{Written for commit b7807bd. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 6 files

_{Re-trigger cubic}