[codex] fix Claude model option merging

[flyinghanger]

Summary

• keep the built-in Claude presets in the composer model picker even when explicit model options are supplied
• merge non-preset Claude model options as additions instead of letting explicit options replace opus / opus[1m]
• add regression coverage for Sonnet-only explicit options and model cycling

Fixes #718.

Validation

• npx --yes bun@1.3.5 run --cwd web test -- AssistantChat/modelOptions.test.ts AssistantChat/claudeModelOptions.test.ts
• npx --yes bun@1.3.5 run --cwd web typecheck
• npx --yes bun@1.3.5 run --cwd web build
• Playwright smoke check against local Vite dev server: Claude options from a Sonnet-only explicit list rendered Default, Sonnet, Sonnet 1M, Opus, and Opus 1M

[github-actions]

Findings
No findings.

Summary
Review mode: initial
No issues found in the changed web/src/components/AssistantChat/modelOptions.ts logic. Residual risk: targeted tests could not be executed in this runner because dependencies are not installed.

Testing
Not run (automation): bun is not installed, and npx --yes bun@1.3.5 run --cwd web test -- AssistantChat/modelOptions.test.ts AssistantChat/claudeModelOptions.test.ts failed because vitest is not installed in this checkout.

HAPI Bot