[rust] Name a oneOf member without a title or type after its base type

[HardMax71]

Fixes #25016.

When an inline oneOf member has no title and no type (for example allOf with a single $ref plus required), the variant name now falls back to the member's base type instead of passing null to toModelName. The type lookup also goes through ModelUtils.getType, which reads types for 3.1 schemas. The repro from #25016 now generates WorkflowPathCondition(Box<models::WorkflowPathCondition>) and the crate compiles, and GitHub's current REST description, which failed on actions-policy-repo-conditions, generates without errors.

The new testOneOfMemberWithoutTitleOrType fails on master with the NullPointerException.

PR checklist

• Read the contribution guidelines.
• Ran ./mvnw clean package and ./bin/generate-samples.sh ./bin/configs/rust-*.yaml; no samples change.
• cc the Rust technical committee: @frol @farcaller @richardwhiuk @paladinzh @jacob-pro @dsteeley

---

Summary by cubic

Fixes a Rust codegen NullPointerException when an inline oneOf member has neither a title nor a type, such as an allOf with a single $ref plus required.

Bug Fixes

• Variant names now fall back to the member's base type instead of passing null to toModelName; type lookup uses ModelUtils.getType, which reads types for 3.1 schemas.
• The repro from [BUG][Rust] NullPointerException naming a oneOf member that has no title or type #25016 now generates WorkflowPathCondition(Box<models::WorkflowPathCondition>) and compiles, and GitHub's current REST description generates without errors.
• Adds testOneOfMemberWithoutTitleOrType, which fails on master with the NullPointerException, and cleans up the files it generates.

^{Written for commit 8a6d013. Summary will update on new commits.}

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[cubic-dev-ai]

All reported issues were addressed across 3 files

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