Handle missing Docker-Content-Digest during manifest resolution

[praneethhere]

Fixes apple/container#1453

Some registries do not return Docker-Content-Digest on manifest HEAD responses. Previously, manifest resolution failed immediately when this header was missing.

This change updates manifest resolution to fall back to a GET request when Docker-Content-Digest is missing, then computes the sha256 digest from the returned manifest body. Existing HEAD-based behavior is preserved when the header is present.

Validation:

• swift build --target ContainerizationOCI

Notes:

• Full swift test is blocked locally by unrelated macOS 26 SDK vmnet symbols.
• make pre-commit is blocked locally by existing swift-fmt parsing errors in ContainerizationExtras address files unrelated to this change.

[jimdigriz]

Misses the Push side

containerization/Sources/ContainerizationOCI/Client/RegistryClient+Push.swift

Lines 157 to 160 in 1437c67

	guard descriptor.digest == response.headers.first(name: "Docker-Content-Digest") else {
	let required = response.headers.first(name: "Docker-Content-Digest") ?? ""
	throw ContainerizationError(.internalError, message: "digest mismatch \(descriptor.digest) != \(required)")
	}