fix(opencode): rescrict github copilot opus 4.7 variants to "medium"

[OpeOginni]

Issue for this PR

Closes #23082

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds case for Github Copilot Provider supporting only the "medium" reasoning variant.

Was able to confirm only this reasoning_effort from the error given to the user on the issue and also using the endpoint https://api.githubcopilot.com/models it return the following for opus 4.7.

        {
            "capabilities": {
                "family": "claude-opus-4.7",
                ...
                "supports": {
                    "reasoning_effort": [
                        "medium"
                    ]
                }
            }
        }

How did you verify your code works?

Ran on my local dev instance and was able to only cycle from default to medium

Screenshots / recordings

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[OpeOginni]

I don't know if the position for the IF is the best, other option would be to put it inside the anthropicAdaptiveEfforts function, but decided to keep it clean, but any suggestion is appriciated. https://github.com/anomalyco/opencode/blob/dev/packages/opencode/src/provider/transform.ts#L392