docs: Update models page with new GPT models and corrected pricing

[morgankrey]

Syncs the models docs page with what crates/billing/src/plans.rs and crates/llm_api/src/features/list_models.rs actually serve.

Changes

New models added:

• GPT-5.4 pro (Pro/Business only): $30/$180 input/output, no cache, 400k context
• GPT-5.4: $2.50/$15 input/output, $0.025 cached, 400k context
• GPT-5.3-Codex: $1.75/$14 input/output, $0.175 cached, 400k context

Pricing corrected:

• GPT-5.2 + GPT-5.2-Codex: input $1.25 → $1.75, output $10 → $14, cached $0.125 → $0.175
• Gemini 3 Flash: input $0.30 → $0.50, output $2.50 → $3.00

Other fixes:

• Student plan note updated to include GPT-5.4 pro restriction
• Grok context windows added to the context window table (128k / 256k for Grok Code Fast 1)
• "GPT-5.2 Codex" renamed to "GPT-5.2-Codex" to match display_name in code
• Removed retired "Gemini 3 Pro" from context window footnote

Release Notes:

• N/A