refactor(plugins/googleai): migrate to v2

[CorieW]

Resolves #3443

Plugin migration guide

Notable Changes

• Migrates from v1 to v2 API.

Testing

• Checked that the same number of modes and embedders load.

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

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

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

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

• Checked that at least one Veo model works consistent with pre-migration on the UI.
  Can't see any load in, even before migration.

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

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

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

• Using an embedder 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.

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

• Using a resolved model object, checked using a flow works.

• Using a resolved embedder object, checked using a flow works.

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

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

• Using a resolved model object, checked using a streaming flow works.

• Tested error response from model.

• Tested error response from embedder.

• Tested model media input.

• Tested model tool calling.

• Tested .list