Fix #56: Correct Callisto client and observatory fallback

[Monier-Ayman]

Description

This PR fixes bug #56 where the Callisto client and attribute names were inconsistent (Callisto vs eCallisto).

Changes made

• spectrogram_factory.py: added a fallback for the observatory field to use "e-CALLISTO" if INSTRUME is missing in the FITS header.
• ecallisto.py: updated post_search_hook to ensure Observatory is always set to "e-CALLISTO" if missing.

Testing

• Verified locally that the fallback works correctly for missing INSTRUME values.
• Confirmed that eCALLISTOClient returns the correct "Observatory" field in search results.
• Tested locally on Windows environment using mock FITS files. Observed observatory fallback works as expected.

This ensures consistent naming and correct behavior when working with eCallisto files in radiospectra.

[Monier-Ayman]

@samaloney Hi! I’ve made the changes for PR #145 to fix bug #56 and standardize the eCALLISTO client and observatory fallback. Could you please take a look when you have a chance and let me know if everything looks good or if you have any feedback? Thanks!