feat: add 1M context support for Claude Sonnet 4.5 via env vars

[stevenvo]

Summary

This PR adds support for enabling 1M token context windows for Claude Sonnet 4.5 through environment variables for both Anthropic direct and Vertex AI Anthropic providers.

Note: Currently only Claude Sonnet 4.5 supports 1M token context windows. All other Claude models (Opus 4, Haiku 4.5, etc.) remain at 200K tokens according to models.dev.

Changes

• Anthropic Direct: Add ANTHROPIC_1M_CONTEXT=true env var to enable 1M context for Sonnet 4.5

  • Preserves existing beta headers (claude-code, interleaved-thinking, fine-grained-tool-streaming)
  • Prepends context-1m-2025-08-07 beta when enabled

• Vertex AI Anthropic: Add VERTEX_ANTHROPIC_1M_CONTEXT=true env var to enable 1M context for Sonnet 4.5

  • Sets anthropic-beta: context-1m-2025-08-07 header when enabled

• Model Detection: Regex pattern specifically matches Claude Sonnet 4.5 variants only

  • Matches patterns: claude-sonnet-4.5, claude-sonnet-4-5, claude-sonnet-4.5-20250929
  • Automatically sets context limit to 1,000,000 tokens when env var is enabled

Testing

• ✅ All existing provider tests pass (66 tests)
• ✅ TypeScript compilation successful
• ✅ No type errors

Usage

```bash

For Anthropic direct (Sonnet 4.5 only)

export ANTHROPIC_1M_CONTEXT=true

For Vertex AI Anthropic (Sonnet 4.5 only)

export VERTEX_ANTHROPIC_1M_CONTEXT=true
```