Support pre-built container image URI in agentcore deploy (skip CodeBuild)

[wilmatos]

Description

Currently, the Container build type always builds images via AWS CodeBuild during agentcore deploy. This doesn't support workflows where users cross-compile for a different architecture, use custom CI/CD pipelines that push images to ECR before deploying, or want to avoid redundant builds when the image already exists in ECR.

Add a --container-uri <uri> flag to agentcore deploy that accepts a pre-built ECR image URI and wires it directly to AgentCoreRuntime, bypassing the CodeBuild step entirely.

Example usage:

agentcore deploy -y --container-uri 123456789012.dkr.ecr.us-west-2.amazonaws.com/myproject/myagent:abc123

The container URI is passed to the CDK app via an environment variable (AGENTCORE_CONTAINER_URI) so the CDK toolkit wrapper doesn't need modification. When set, the CDK stack creates an AgentCoreRuntime construct directly with the pre-built image instead of going through the AgentCoreApplication (CodeBuild) path.

Acceptance Criteria

• agentcore deploy --container-uri <uri> deploys using the provided ECR image without triggering a CodeBuild build
• Invalid ECR URI formats are rejected with a clear error message before deployment starts
• Dockerfile validation is skipped when --container-uri is provided
• The flag triggers non-interactive (CLI) mode
• DeployOptions and ValidatedDeployOptions interfaces include the optional containerUri field
• The CDK stack branches correctly: containerUri set → AgentCoreRuntime with pre-built image; unset → existing AgentCoreApplication (CodeBuild) path
• CfnOutput for RuntimeId and RuntimeArn is emitted in the pre-built image path
• Unit tests cover: URI validation (valid/invalid formats), Dockerfile validation skip, env var propagation, CDK stack branching logic
• Existing Container build type (no --container-uri) continues to work unchanged

Additional Context

This currently assumes a single agent (spec.agents[0]). We likely would want to address multi-agent/multi-URI mappings somehow.
The ECR repo must already exist with the referenced tag.