Implement configurable default namespace for model references

[Copilot]

This PR implements configurable default namespace support for container registry references in the model-distribution system, addressing the issue described in docker#126.

Problem

Currently, the system uses github.com/google/go-containerregistry/pkg/name package directly, which defaults to Docker Hub (index.docker.io) for references without explicit registry hostnames. The upstream library doesn't provide a configurable default namespace option, making it difficult to use alternative default registries for model distribution.

Solution

This PR introduces a new internal/naming package that wraps the upstream parsing functions with configurable default namespace support. The implementation:

1. Preserves backward compatibility - existing code continues to work unchanged
2. Provides flexible configuration - supports client-specific, global, and CLI configuration
3. Uses intelligent registry detection - distinguishes between tags (mymodel:latest) and registry ports (localhost:5000/model)

Key Changes

New Naming Package

• Created DefaultNamespace struct with configurable registry
• Implemented ParseReference() and ParseTag() wrapper methods with smart registry detection
• Added global configuration functions and comprehensive test coverage

Registry Client Enhancement

// New option for configurable default namespace
client := registry.NewClient(registry.WithDefaultNamespace("registry.example.com"))

Distribution Client Enhancement

// Unified configuration across registry and store operations
client, err := distribution.NewClient(
    distribution.WithStoreRootPath("/path/to/store"),
    distribution.WithDefaultRegistry("registry.example.com"),
)

CLI Tool Enhancement

# New --default-registry flag
mdltool --default-registry registry.example.com pull mymodel:latest
# Resolves to: registry.example.com/mymodel:latest instead of index.docker.io/library/mymodel:latest

Behavior Examples

Input	Default Registry	Output
mymodel:latest	registry.example.com	registry.example.com/mymodel:latest
user/mymodel:latest	registry.example.com	registry.example.com/user/mymodel:latest
other.registry.com/model:latest	registry.example.com	other.registry.com/model:latest (preserved)
mymodel:latest	(none)	index.docker.io/library/mymodel:latest (Docker Hub default)

Registry Detection Logic

The package uses heuristics to detect explicit registries:

• mymodel:latest → No explicit registry (:latest is a tag)
• registry.com/mymodel:latest → Has explicit registry (contains dot)
• localhost:5000/mymodel:latest → Has explicit registry (port number)

Integration Points

All reference parsing throughout the codebase now consistently uses the configurable namespace:

• Registry client operations (Model, BlobURL, BearerToken, NewTarget)
• Store operations (Tag, UnTag, HasTag, MatchesReference)
• CLI commands (pull, push, tag, etc.)

Testing

Added comprehensive test coverage including:

• Unit tests for namespace parsing logic
• Integration tests for client configuration
• CLI tests for command-line flag functionality
• Edge cases for registry detection heuristics

This change provides the exact functionality requested in docker#126 - a way to configure default namespaces without requiring upstream changes to go-containerregistry.

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