Add fgco2 and volcello to ESM1.6 CMIP7 baseline batch config

[rbeucher]

Summary

• fgco2 and volcello already had correct, working mappings in ACCESS-ESM1-6_mappings.json, but neither was listed in any of the places that actually request variables for a CMORisation run, so no run ever produced them.
• Adds ocnBgchem.fgco2.tavg-u-hxy-sea.mon.glb (→ Omon.fgco2) and ocean.volcello.tavg-ol-hxy-sea.mon.glb (→ Omon.volcello) to:
  • the variables: list in batch_config_esm1-6_cmip7_baseline.yml (example config)
  • the embedded CONFIG_TEMPLATE in scripts/generate_configs.py (the generator used to produce real per-experiment batch configs for CMIP7 fast-track ensemble runs — it has its own copy of the variable list and had the same gap)
• Gives volcello the same large_ocean resource override (256GB mem, 7 cpus) as other full 3D (time, lev, lat, lon) monthly ocean fields like thkcello/thetao, in both files. fgco2 is a 2D surface flux (like hfds) and needs no override.

Fixes #631

Test plan

• Verified ocnBgchem.fgco2.tavg-u-hxy-sea.mon.glb and ocean.volcello.tavg-ol-hxy-sea.mon.glb resolve end-to-end via _get_cmip7_to_cmip6_mapping → Omon.fgco2 / Omon.volcello, load_model_mappings finds each variable in ACCESS-ESM1-6_mappings.json, and CMIP7Vocabulary parses the CMOR table for both without error.
• yaml.safe_load on both the updated example config and the rendered CONFIG_TEMPLATE confirms both variables and the new resource override are present in each.
• Confirmed the fx.areacello_ACCESS-ESM.nc bundled resource file used by volcello's calculation exists in the package.
• Ran the relevant integration tests for both variables; the only failures reproduce identically on unmodified main (pre-existing local CMIP6Plus_CVs submodule vocabulary lookup issue, unrelated to this change).

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.2%. Comparing base (b16cd01) to head (d75163c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #640   +/-   ##
=====================================
  Coverage   77.2%   77.2%           
=====================================
  Files         40      40           
  Lines       8514    8514           
  Branches    1590    1590           
=====================================
  Hits        6575    6575           
  Misses      1610    1610           
  Partials     329     329           

Flag	Coverage Δ
unit	77.2% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.