refactor: change DataStore::get_note_script to return Option<NoteScript>

[partylikeits1983]

This PR resolves: #2029

Follow up PR to #1995

This PR changes the DataStore::get_note_script API to return Result<Option<NoteScript>, DataStoreError>
instead of Result<NoteScript, DataStoreError>. This makes the distinction between "script not found" (represented by Ok(None)) and "error retrieving data" (represented by Err) explicit.

Benefits:

• Clearer intent: The API signature explicitly communicates that "not found" is a valid, expected outcome
• Simpler error handling: Consumers can use ? to propagate internal errors while handling None separately
• Better separation of concerns: Distinguishes between "data not present" and "error retrieving data"
• Follows Rust idioms where Option represents presence/absence and Result represents success/failure

[PhilippGackstatter]

Looks good to me!

[bobbinth]

Looks good! Thank you!