bug(ecs): fix parseContainerImage to handle images with registry ports

[zyzzmohit]

What this PR does:
Updates parseContainerImage inside pkg/app/piped/platformprovider/ecs/task.go to use strings.LastIndex for separating the container image tag from the image name. It also adds unit tests to TestFindArtifactVersions for explicitly testing images with registry ports.

Why we need it:
The previous implementation naively split the image string by : to extract the image tag. If a user relies on a container image hosted in a registry that specifies a port (e.g., localhost:5000/repo/image:tag), the function incorrectly split the URI, resulting in a misparsed name and an empty tag.

This is the exact same architectural flaw recently identified and fixed in the Cloud Run provider (pkg/app/piped/platformprovider/cloudrun/servicemanifest.go). Because AWS ECS frequently utilizes custom Docker registries with ports, this is a necessary defensive fix mapping to real-world use cases.

Which issue(s) this PR fixes:

Fixes #6575

Does this PR introduce a user-facing change?:
Yes

• How are users affected by this change: Users can now safely configure their ECS task definitions to use container images hosted in external registries containing custom ports (e.g., my-registry.com:8443/app:v1).
• Is this breaking change: No
• How to migrate (if breaking change): N/A