feat(conversation): enable update mutations on conversation model

[atierian]

Description of changes

• Enables model generated update mutation on Conversation<RouteName> model type for conversation routes.
• Adds E2E test for update conversation mutation.

Related PRs

• codegen: feat: remove update: null for conversation model in MIS amplify-codegen#895
• data-schema: feat(ai): add update conversation amplify-data#365

Release ordering:
1./2. amplify-category-api and amplify-codegen. Note: Ideally amplify-category-api first, but it's not entirely necessary.
3. amplify-api-next. This must go last to ensure the update API in the data client hits an existing GraphQL mutation field.

CDK / CloudFormation Parameters Changed

N/A

Issue #, if available

N/A

Description of how you validated changes

• CI PR Workflow
• Added E2E test -- E2E run

Checklist

• PR description included
• yarn test passes
• E2E test run linked
• Tests are changed or added
• Relevant documentation is changed or added (and PR referenced)
• New AWS SDK calls or CloudFormation actions have been added to relevant test and service IAM policies
• Any CDK or CloudFormation parameter changes are called out explicitly

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.