fix(LAC-3589): remove retired Claude models from provider definitions

[lacymorrow]

Weekly Model Update Check — Juno (LAC-3589)

Checked src-tauri/src/agent/providers/types.rs against the live Anthropic model-deprecations and models-overview pages.

Finding: three models in the legacy tier are now retired (return 404)

Verified against the live model-deprecations page:

Model ID	Status	Retired
claude-opus-4-1-20250805	Retired	2026-08-05
claude-opus-4-20250514	Retired	2026-06-15
claude-sonnet-4-20250514	Retired	2026-06-15

Requests to retired models fail with 404, so leaving them in the selector let a user pick a model that hard-fails on every request. This PR removes their model_ids constants and ModelDefinition entries.

Changes

• Remove CLAUDE_OPUS_4_1, CLAUDE_OPUS_4, CLAUDE_SONNET_4 constants + their ModelDefinition entries.
• Re-point the two Tier-3 passthrough tests at a literal retired ID (claude-3-5-sonnet-20241022) — no active model exercises that branch anymore, and the removed constant is no longer available.
• Add a maintenance note pointing future editors at the live deprecations page before re-adding any ID.

Verified still correct (no change needed)

• Current active lineup matches: Fable 5 / Opus 5 (recommended default) / Sonnet 5 / Haiku 4.5.
• Legacy-but-callable all retained: Opus 4.8/4.7/4.6/4.5, Sonnet 4.6/4.5.
• Frontend: zero hardcoded claude-* IDs (selectors fetch via invoke("get_provider_models")), so consistency is automatic.

Verification

• rustfmt --check on types.rs: clean
• cargo test --lib agent::providers::types: 13/13 pass (incl. test_opus_5_is_default_anthropic_model)
• No remaining backend references to the removed constants (grep clean → no compile breakage)

🤖 Generated with Claude Code