Add latest models across all providers with legacy flag support

[hershkop]

Summary

• Added legacy flag to OpenAI, Anthropic, and Google model types — legacy models sort to the bottom of dropdowns while remaining fully functional for existing graphs
• Added new OpenAI models: gpt-5.1, gpt-5.2, gpt-5.5, gpt-4.1-mini, gpt-4.1-nano, o3-pro
• Added new Anthropic models: claude-haiku-4-5-20251001, claude-sonnet-4-6-20260414, claude-opus-4-6-20260414
• Added new Google Gemini models: gemini-3-1-pro-preview, gemini-3-pro-preview, gemini-3-flash-preview
• Marked legacy models: gpt-4o, gpt-4o-mini, o1, o1-mini, claude-instant-1/1.2, claude-2/2.1, claude-3-haiku/sonnet/opus, gemini-1.5-pro/flash
• Fixed bug: gpt-5-mini prompt cost was 0.25 - 6 (subtraction) instead of 0.25e-6
• Added model definition tests covering required fields, no duplicates, cost validation, and legacy sorting

Test plan

• TypeScript builds clean (yarn workspace @ironclad/rivet-core run build)
• All 75 tests pass (yarn workspace @ironclad/rivet-core run test) — 12 new model tests + 63 existing
• Lint passes with 0 errors (yarn lint)
• Verify new model IDs work against live APIs using temptest/ scripts (not committed — run locally with API keys)
• Manually confirm dropdown ordering in the app — new models at top, legacy at bottom

🤖 Generated with Claude Code