fix: drop OpenAI models shut down on Oct 23 and move the default off Sonnet 4.5

[volodymyrshk]

Same change as e2b-dev/fragments#256. OpenAI shuts down o3-mini, o4-mini, gpt-4.1-nano and gpt-4-turbo on October 23, 2026, and Anthropic retires claude-sonnet-4-5-20250929, the default model here, on November 30 (https://developers.openai.com/api/docs/deprecations, https://platform.claude.com/docs/en/about-claude/model-deprecations).

Changes:

• Remove the four OpenAI entries from lib/models.json.
• Replace claude-sonnet-4-5-20250929 with claude-sonnet-4-6 and make it the default in components/analyst.tsx.

Why 4.6 and not Sonnet 5.5: the app is on ai 3.4.x, which sends temperature: 0 when none is set, and Sonnet 5.5 rejects a non-default temperature with a 400. Sonnet 4.6 accepts it and can't retire before February 17, 2027.

A saved selection that is no longer in models.json already falls back to the default in analyst.tsx.

npm test: 76 passed. tsc --noEmit: clean.

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[mishushakov]

please sign the CLA and we'd be happy to move forward