chore(release): prepare 0.18.0 - #84

Release

• Bump openapi-to-rust and its lockfile to 0.18.0.
• Document the generated Rust type changes from nullable union properties and the wire-equivalent union deserialization fixes.
• Update changelog comparison links.

This minor release includes merged PRs #82 and #83. Generated model field types can change when a union branch admits JSON null; consumers should regenerate their clients and update affected constructors or field access.

Verification

• All release PR checks passed: tests, format, clippy, docs, install smoke, MSRV, SDK compatibility, corpus diff, and spec compile smoke.
• PR fix(generator): preserve union input and nullable results #83 passed the local full spec compile: 55 specs passed and Microsoft Graph compiled separately. The hosted full spec job still exits 143 while processing Cloudflare; tracked as openapi-generator-bh9.