chore: prepare 0.2.3 release

[adamcavendish]

Summary

Prepare the 0.2.3 release containing plain internally tagged oneOf detection
and add explicit Just recipes for publishing openapi-nexus to crates.io.

Problem

The tagged-union lowering fix is merged, but the workspace version, changelog,
and stable installer documentation still identify 0.2.2. Publishing the crate
also requires invoking Cargo directly instead of using the repository's task
interface.

Behavior

Surface	0.2.3 behavior
Workspace packages	Report version 0.2.3
Changelog	Publishes plain internally tagged oneOf detection and the new publish recipes
Stable installer links	Download release 0.2.3
Package preflight	just publish-dry-run verifies the openapi-nexus crate
crates.io publication	just publish publishes only the openapi-nexus crate

Compatibility

This release preparation adds no new Rust public API. Generated clients now
recognize plain inline internally tagged oneOf variants across OpenAPI 3.0,
3.1, and 3.2, preserving nested TypeScript camel-case field conversion. The
publish recipes are maintainer-facing and keep fixture crates out of the
crates.io operation.

Validation

• cargo publish --dry-run --allow-dirty -p openapi-nexus
• just fmt-check
• just clippy
• just doc-check
• just book
• just msrv
• just test-all with JDK 21: 1,308 tests passed
• just coverage-lcov with JDK 21: 1,308 tests passed and report generated
• just golden-check: 569 golden tests passed
• git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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