Extract manifest path resolution logic into helper function

[coderabbitai]

Description

The manifest path resolution and creation logic in the run() function within tests/steps/process_steps.rs is verbose and inline, reducing maintainability. This logic should be extracted into a dedicated helper function for better code organisation.

Current Problem

The current implementation contains approximately 23 lines of inline logic for:

• Resolving absolute vs relative manifest paths
• Creating temporary manifest files when they don't exist
• Persisting the manifest files
• Updating the CLI configuration

Proposed Solution

Extract this logic into a helper function ensure_manifest_exists in the support module that:

• Takes temporary directory path and CLI file path as parameters
• Handles absolute/relative path resolution
• Creates manifest files when missing
• Returns the resolved manifest path
• Allows the main run() function to be simplified

References

• Original PR: Handle manifest path in process tests #56
• Discussion: Handle manifest path in process tests #56 (comment)
• Issue: Extract manifest path resolution logic into helper function #59
• Requested by: @leynos