Improve Cursor compatibility: model alias routing + robust multimodal input flattening

[Copilot]

Cursor can intercept first-party model names (e.g. gpt-4o), bypassing custom gateway routing, and its multimodal payloads can break proxy flows that stringify structured content. This change adds Cursor-safe model aliases and normalizes image-bearing content paths to preserve stable request handling.

• Model routing: Cursor-safe aliases

  • Expose copilot-* aliases in GET /v1/models (e.g. copilot-gpt-4o) mapped to discovered VS Code LM models.
  • Extend model lookup normalization to resolve prefixed variants (copilot-, cursor-, cursor/, openai/) and normalized family/id forms.

• Multimodal/vision content handling

  • Add shared compatibility flattening for OpenAI/Cursor-style structured content blocks.
  • Handle image, image_url, and input_image parts explicitly as [image omitted] placeholders instead of unsafe JSON stringification.
  • Apply the same flattening path in both Responses API sync and streaming input parsing.

• Compatibility utilities + docs

  • Introduce src/compatibility.ts for model-key normalization, alias generation, and content flattening.
  • Update Cursor docs to recommend selecting copilot-* models from /v1/models to avoid model-name interception.

// Cursor/OpenAI-prefixed names now normalize to the same lookup key
normalizeModelLookupKey('cursor/gpt-4o-20241022') // -> "gpt-4o"
normalizeModelLookupKey('copilot-gpt-4o')         // -> "gpt-4o"

// Structured image blocks are safely flattened
flattenOpenAICompatibleMessageContent([
  { type: 'text', text: 'Analyze this' },
  { type: 'image_url', image_url: { url: 'data:image/png;base64,...' } }
])
// -> "Analyze this\n[image omitted]"

• Fixes Not work in cursor #160