Add Claude Opus 4.6 (1M context) for GitHub Copilot

[zhzy0077]

Seems to be available soon: github/copilot-cli#1395

[nkhoit]

For reference I have access to this so the metadata API response for claude-opus-4.6-1m from GH CP CLI looks like this:

{
  "billing": {
    "is_premium": true,
    "multiplier": 6,
    "restricted_to": [
      "pro",
      "edu",
      "pro_plus",
      "business",
      "enterprise"
    ]
  },
  "capabilities": {
    "family": "claude-opus-4.6-1m",
    "limits": {
      "max_context_window_tokens": 1000000,
      "max_non_streaming_output_tokens": 16000,
      "max_output_tokens": 64000,
      "max_prompt_tokens": 936000,
      "vision": {
        "max_prompt_image_size": 3145728,
        "max_prompt_images": 1,
        "supported_media_types": [
          "image/jpeg",
          "image/png",
          "image/webp"
        ]
      }
    },
    "object": "model_capabilities",
    "supports": {
      "adaptive_thinking": true,
      "max_thinking_budget": 32000,
      "min_thinking_budget": 1024,
      "parallel_tool_calls": true,
      "streaming": true,
      "structured_outputs": true,
      "tool_calls": true,
      "vision": true
    },
    "tokenizer": "o200k_base",
    "type": "chat"
  },
  "id": "claude-opus-4.6-1m",
  "is_chat_default": false,
  "is_chat_fallback": false,
  "model_picker_category": "powerful",
  "model_picker_enabled": true,
  "name": "Claude Opus 4.6 (1M context)",
  "object": "model",
  "policy": {
    "state": "enabled",
    "terms": "Enable access to the latest Claude Opus 4.6 (1M context) model from Anthropic. [Learn more about how GitHub Copilot serves Claude Opus 4.6 (1M context)](https://gh.io/copilot-claude-opus)."
  },
  "preview": false,

[nkhoit]

I think from the API response (#897 (comment)) it should be

context = 1_000_000
input = 936_00
output = 64_000