feat: actionable error messages for unsupported Git hosts

[danielmeppiel]

• Add unsupported_host_error() function with clear fix instructions
• Show supported hosts list (github.com, *.ghe.com, Azure DevOps)
• Include platform-specific env var examples (Linux/macOS/Windows)
• Display mismatch when GITHUB_HOST is set but different host is used
• Add unit tests for new error message function

[Copilot]

Pull request overview

This PR introduces a centralized, actionable error message for unsupported Git hosts and wires it into the dependency parsing logic, so users get clear guidance when they hit host validation failures.

Changes:

• Added unsupported_host_error() in github_host.py to generate a detailed message listing supported hosts and platform-specific GITHUB_HOST configuration examples, including showing the current GITHUB_HOST when it doesn’t match the attempted host.
• Updated DependencyReference.parse() in apm_package.py to use unsupported_host_error() for unsupported-host failures, ensuring consistent messaging across hostname validation paths.
• Extended unit tests in tests/unit/test_github_host.py to cover the new error helper, including content checks (supported host list, env var instructions, and current host mismatch behavior).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
tests/unit/test_github_host.py	Adds tests verifying unsupported_host_error() contents and behavior when GITHUB_HOST is set, improving coverage of the new helper.
src/apm_cli/utils/github_host.py	Introduces unsupported_host_error() and its user-facing message, enumerating supported hosts and including platform-specific GITHUB_HOST setup instructions.
src/apm_cli/models/apm_package.py	Integrates unsupported_host_error() into DependencyReference.parse() for unsupported-host failures, aligning security checks with clearer, centralized error messaging.