fix(ai): project FixedBroker scalar unions for Codex

[first-assist]

Summary:

• preserve the bounded nullable scalar type arrays used by the three crate-authored FixedBroker definitions; Codex 0.148.0 accepts this exact form
• admit only unique nullable combinations of string, integer, number, boolean, and null, with compatible enums and constraints; reject structural, unknown, duplicate, and non-nullable unions
• regression-test the actual discover, describe, and execute definitions and release graphql-orm-ai 0.88.1 without changing the 0.63.0 schema module

Documentation impact:

• Documentation updated
• No documentation impact

Verification:

• cargo check -p graphql-orm-ai --features provider-codex-app-server
• full all-provider test matrix: 392 passed, 3 ignored, all integration suites passed
• crate-scoped all-target warnings-denied Clippy passed with --no-deps
• warnings/missing-docs-denied Rustdoc passed
• PascalCase naming test and Rustdoc passed
• PostgreSQL and MSSQL compile-only lanes passed
• documentation validation passed for 187 governed Markdown files; fmt and diff checks passed
• release policy and cargo-semver-checks passed against 4f903aa

Baseline note:

• the dependency-inclusive Clippy command reaches a pre-existing graphql-orm-macros collapsible_else_if lint at operations.rs:4870. Main fixed that line inside PR feat(router): configure generic scope matching #19, but backporting feat(router): configure generic scope matching #19 would also widen the type universe to agql-auth 0.16. This maintenance PR deliberately does not do that; hosted CI is the final gate.