fix(ai): restore retained FixedBroker Codex admission

[first-assist]

Summary

• port the reviewed nullable scalar-union Codex projection from the 0.88 maintenance line onto auth-0.18 main
• port exact capability-session binding admission for retained Codex create, resume, discard, and cleanup
• release graphql-orm-ai 0.91.1 with aligned migration, changelog, README, lockfile, and generated workspace inventory

This is required before the GEMA fame-ai consumer can move from graphql-orm-ai 0.88.3 / agql-auth 0.16 to the current 0.91.x / agql-auth 0.18 type universe. Removing the consumer builder call is not safe: main at 8c19dbf rejects the complete capability-binding fingerprint as though it were a raw registration fingerprint.

Verification

• bounded cargo fmt --all -- --check
• bounded full graphql-orm-ai provider-feature test matrix, including provider-codex-app-server
• bounded warnings-denied clippy and rustdoc
• PascalCase GraphQL test and rustdoc lane
• compile-only PostgreSQL and MSSQL lanes
• package release policy and cargo-semver-checks against origin/main
• documentation, generated inventory, workspace dependency, release-state, and release-manifest checks
• dependency tree resolves one agql-auth 0.18.0 source at 527d15d2

The all-provider test build was rerun with a command-local one-job override after two simultaneous graphql-orm-ai codegens were measured thrashing at MemoryHigh. Repository Cargo configuration and the shared eight-job default are unchanged.

Documentation impact

• Documentation updated
• No documentation impact

README, changelog, migration guidance, provider-session guidance, and generated package inventory are updated. There is no database, schema-module, GraphQL SDL, protected-payload, backup, or restore migration.