fix: reject ambiguous implicit discriminators

[lightsofapollo]

Summary

• require inferred discriminator values to be unique across every union branch
• fall back to an untagged union when repeated values would create duplicate Serde tags
• add the live OpenCode OpenAPI document to the generation and compile corpus
• add regression coverage for ambiguous and unique inferred discriminators

Validation

• cargo test --all-features
• cargo clippy --all-features -- -D warnings
• cargo fmt -- --check
• SPEC_COMPILE_PARSE_ONLY=1 scripts/spec-compile.sh
• SPEC_COMPILE_KEEP=1 scripts/spec-compile.sh opencode
• generated OpenCode GlobalEvent runtime round-trip for a nested sync event

Closes #54

[vercel]

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Project	Deployment	Actions	Updated (UTC)
openapi-to-rust	Ready	Preview	Aug 7, 2026 5:59pm