Add Japan (392) to the offline demographics country map

[arihantlodha-cmd]

Adds "392": "JPN" to the country_dict in ogcore/demographics.py, so the offline data fallback (used when no UN API token is present) can find Japan's demographic data by country code, like the other supported countries.

This pairs with EAPD-DRB/Population-Data#14, which adds the Japan data (Data/JPN/) to the mirror this fallback reads from. That PR should merge first; on its own this entry has no data behind it yet.

Motivation: I'm building an open-source OG model of Japan on top of OG-Core, and Japan is currently the only major economy without an OG country model. This (plus the data PR) lets OG-Core use Japan demographics offline. With a UN API token the live pull already works for Japan (code 392); this just extends the token-free path.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.74%. Comparing base (a8b0232) to head (bb8831e).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1196   +/-   ##
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  Coverage   72.74%   72.74%           
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  Files          22       22           
  Lines        5768     5768           
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  Hits         4196     4196           
  Misses       1572     1572           

Flag	Coverage Δ
unittests	72.74% <ø> (ø)

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Files with missing lines	Coverage Δ
ogcore/demographics.py	58.50% <ø> (ø)

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[marcelolafleur]

Thanks @arihantlodha-cmd — the entry looks right. One note for whoever merges: Data/JPN doesn't exist on the Population-Data side yet, so this would 404 today. Being sorted in EAPD-DRB/Population-Data#14.

Also, while you're in this dict — worth adding "242": "FJI". Fiji landed in the archive this morning and this map missed it, so the offline path KeyErrors on Fiji right now.