Initial draft for metadata factory migration

[Amityush-lgtm]

PR Description

For now, this draft PR starts the metadata factory migration by moving Callisto and WAVES to the new from_raw() pattern.
This is just an initial step to understand the migration, @samaloney and @hayesla Let me know if this looks like the right direction.

Changes:

• Factory now returns raw (header, raw_object) pairs for Callisto and WAVES.
• Moved parsing logic into from_raw() in the instrument classes.
• Added compatibility for both the old and new factory flow.
• Updated the tests.

AI Assistance Disclosure

AI tools were used for:

• Code generation (e.g., when writing an implementation or fixing a bug)
• Test/benchmark generation
• Documentation (including examples)
• Research and understanding
• No AI tools were used

Regardless of AI use, the human contributor remains fully responsible for correctness, design choices, licensing compatibility, and long-term maintainability.