Add standard Sonnet 4.6 to VSCode extension model picker

The VSCode extension model selector doesn't offer standard Sonnet 4.6 as an option. On the Max plan, the available choices are:

• Default (labeled "Sonnet 4.6" but actually resolves to Opus 4.6)
• Sonnet 4.6 with 1M context (billed as extra usage)
• Opus 4.7 (1M context)
• Haiku 4.5
• Opus 4.6

Two issues:

1. There's no way to select standard Sonnet 4.6 (non-1M context) — the same model that's available on claude.ai
2. The "Default (recommended)" label says "Sonnet 4.6" but actually sets the model to Opus 4.6, which is misleading

Expected: Standard Sonnet 4.6 should be available as a model option, and the Default label should accurately reflect which model it resolves to.