fix(codecs): correct wavelength padding key in pad_spectrum

[hopeful0]

While using the SDSS spectrum codec, I kept seeing failures consistent with #61 even after #64 landed. Tracing the encode path showed that pad_spectrum (introduced in #64) pads flux, ivar, mask, and wavelength, but padding_values still used the key "lambda". Because the modality field is wavelength, those padded positions were filled with 0 instead of the intended 99999 sentinel.

This change updates the dict key to "wavelength" so padded wavelength bins match the other fields.

[hopeful0]

It seems the issue is that pad_spectrum uses right padding, and padding wavelenght with 0 disrupts the monotonic increasing property of the wavelengths, causing encoding errors

[tom-hehir]

Hi @hopeful0, thank you very much for identifying this! We also ran into this issue and just pushed a fix on commit 3434e31 (latest commit on main). Apologies that we only saw this issue after pushing the fix, otherwise we could have just merged this PR! Thank you for sending in the fix anyway :)