fix(models): match opencode Codex model filter

[rexdotsh]

Summary

• Match the Codex/ChatGPT OAuth model filtering behavior from opencode dev, pinned to commit 9976269ab1accfc9f9dc98a4a688c516934de422
• Add dynamic inclusion for GPT model versions greater than 5.4, including current gpt-5.6 variants
• Explicitly exclude gpt-5.5-pro and update model registry contract coverage

Tests

• bun test
• bun typecheck
• bun lint