refactor(plugins/anthropic): migrate plugin to v2 API

[CorieW]

Resolves#349

What this does

• Upgrades Genkit to the latest version
• Migrates the plugin API to v2 (doc)

Testing

• Checked that the models load with the same names/prefixes on the UI.

• Checked that at least one model works consistent with pre-migration on the UI.

• Using a model object, checked using a flow works consistent with pre-migration.

• Using a model string ('pluginName/modelName'), checked using a flow works consistent with pre-migration.

• Tested error response from model.

• Tested resolve.

• Tested model streaming in flow.

• Tested model media input.

• Tested model tool calling.

• Tested .list

[cabljac]

@CorieW build is failing on anthropic example, probably needs a bit of investigation

[cabljac]

couple of comments so far

[cabljac]

I think the list method might need to call the anthropic SDK list models method/ API endpoint, and return "potential" models etc. We should double check this. This seems to be what google-genai is doing.

[cabljac]

is comment necessary? would rather have a comment explaining why this logic works/is necessary

[cabljac]

or no comment maybe, clear from code above