Implement fillvalue support for History (instantaneous)

[jimmielin]

Tag name (required for release branches):
Originator(s): @jimmielin co-authored with claude-opus-4.6

Description (include the issue title, and the keyword ['closes', 'fixes', 'resolves'] followed by the issue number):

• Implements writing _FillValue and missing_value to history netCDF files (instantaneous)
• Fix flag_xyfill was not being passed down to call hist_new_field

Describe any changes made to build system: N/A

Describe any changes made to the namelist: N/A

List any changes to the defaults for the input datasets (e.g. boundary datasets): N/A

List all files eliminated and why: N/A

List all files added and what they do: N/A

List all existing files that have been modified, and describe the changes:
(Helpful git command: git diff --name-status development...<your_branch_name>)

M       src/history/cam_hist_file.F90
  - correctly pass down flag_xyfill to hist_new_field
  - write _FillValue and missing_value (float32 or float64) to history file

M       src/history/cam_history.F90
  - update todo comment that flag_xyfill still to be handled for accumulated fields

If there are new failures (compared to the test/existing-test-failures.txt file),
have them OK'd by the gatekeeper, note them here, and add them to the file.
If there are baseline differences, include the test and the reason for the
diff. What is the nature of the change? Roundoff?

derecho/intel/aux_sima:

  SMS_Ln9.ne3pg3_ne3pg3_mg37.FKESSLER.derecho_intel.cam-outfrq_se_cslam_multitape (Overall: NLFAIL) details:
    FAIL SMS_Ln9.ne3pg3_ne3pg3_mg37.FKESSLER.derecho_intel.cam-outfrq_se_cslam_multitape NLCOMP
  - pre-existing failure

derecho/gnu/aux_sima:

  SMS_Ln9.ne3pg3_ne3pg3_mg37.FADIAB.derecho_gnu.cam-outfrq_se_cslam (Overall: FAIL) details:
    FAIL SMS_Ln9.ne3pg3_ne3pg3_mg37.FADIAB.derecho_gnu.cam-outfrq_se_cslam RUN time=9
  - pre-existing failure
  - this is a suspect test, it worked in the last PR but not this one and is expected to fail.
  - expected to be resolved with the SE dycore PR

derecho/nvhpc/aux_sima: All PASS

If this changes climate describe any run(s) done to evaluate the new
climate in enough detail that it(they) could be reproduced:

CAM-SIMA date used for the baseline comparison tests if different than latest:

[peverwhee]

thanks @jimmielin !

[nusbaume]

Thanks again @jimmielin!