Add stage field to JobError model

[Copilot]

This PR adds support for the new stage field in the JobError model to align with changes made in Home Assistant Supervisor PR #5784.

The Supervisor now includes a stage field in job error responses to indicate what stage of a job an error occurred in. This allows for better error tracking and diagnostics, particularly useful for distinguishing between errors that occur during different phases of operations like backup creation vs. copying to additional locations.

Changes Made

• Added stage: str | None = None field to the JobError dataclass
• Updated test fixtures to include the stage field in error objects
• Added test assertions to verify the stage field is properly parsed
• Created backward compatibility test to ensure the client works with legacy error responses that don't include the stage field

Backward Compatibility

The stage field is optional with a default value of None, ensuring full backward compatibility with existing Supervisor versions that don't include this field in error responses.

Example usage:

# New format with stage information
error = JobError(
    type="BackupError", 
    message="Could not copy backup", 
    stage="copy_additional_locations"
)

# Legacy format still works (stage defaults to None)
error = JobError(type="BackupError", message="Could not copy backup")

Fixes #166.

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