Show Gemini 4 Argon in model picker

[damienrj]

Summary

• Display Google's gemini-4-argon as Gemini 4 Argon in the model picker.
• Give the Goose-prefixed goose-gemini-4-argon model the same name.
• Add coverage for both discovered model IDs.

The Google provider discovers models dynamically, so this does not add a static model ID or change provider defaults.

Validation

• just check
• just test (8,444 passed, 1 skipped)
• Push hooks: fmt-check, check, tauri-check, and clippy

[morgmart]

🤖 Automated code review

APPROVE. The exact three-dot comparison correctly gives the discovered Gemini 4 Argon model its official display name across Google and Goose-backed provider paths, with discriminating coverage and no publishable findings. The supplied GitHub evidence is structurally valid: five checks completed successfully and four checks remain in progress, so required checks still govern merge readiness.

Deterministic publication result: 0 blocking and 0 non-blocking inline finding(s) publishable; 0 duplicate(s) suppressed; 0 blocking screenshot-evidence requirement(s) in this review body.

Pending checks: 3 check(s) are not complete.

This approval reflects the completed code review only; merge readiness remains governed by the repository's required checks.