Fix CI: pin GAMS 53.5.1 + robust pTransferLimit season check (#125)

* fix: only check existing zone pairs in pTransferLimit season validation

The seasonal-coverage check in _check_transfer_limits grouped pTransferLimit
records with observed=False. Once the GAMS Python API began returning the
z/z2 columns as categorical dtype (recent gams/gams image), observed=False
started enumerating the full cartesian product of zones, creating empty
phantom groups for pairs that have no transfer limit. Those empty groups were
flagged as missing all seasons, failing CI on unchanged data.

Use observed=True so only zone pairs actually present in pTransferLimit are
validated. Real incomplete pairs are still caught.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* ci: pin GAMS image to 53.5.1 (last known-good)

GAMS 54.1.0 changed gams-transfer to return categorical dtypes, which broke
the pipeline in multiple places (phantom groupby groups in input_verification,
UNDF values in input_treatment generic defaults). Pin to 53.5.1 — the version
that was green on June 3 — to restore CI. Revisit 54.x compatibility separately.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* docs: note GAMS 53.x requirement / 54.x unsupported (refs #126)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: mbaronnet

.github/workflows/gams_workflow.yml

@@ -10,7 +10,10 @@ jobs:
   build:
     runs-on: ubuntu-latest
     container:
-      image: gams/gams:latest
+      # Pinned: GAMS 54.x returns gams-transfer records with categorical dtypes,
+      # which broke input verification/treatment (phantom groupby groups, UNDF
+      # values). Last known-good is 53.5.1. Re-evaluate before moving to :latest.
+      image: gams/gams:53.5.1
 
     steps:
       - name: Check out repository

README.MD

@@ -69,6 +69,8 @@ Install EPM and choose how you want to run it:
 
 Full installation instructions are available [here](https://esmap-world-bank-group.github.io/EPM/run/run_installation/).
 
+> **GAMS version:** EPM is tested on **GAMS 53.x** (CI pinned to 53.5.1). **GAMS 54.x is not yet supported** — it changes data types returned by `gams-transfer` and breaks input processing. Please use GAMS < 54 for now. Tracking: [#126](https://github.com/ESMAP-World-Bank-Group/EPM/issues/126).
+
 ---
 
 ## Works deploying EPM

epm/input_verification.py

@@ -656,7 +656,7 @@ def _check_transfer_limits(gams, db):
         seasons = set(p_hours_records["q"].unique())
         season_issues = []
 
-        for (z, z2), group in records.groupby(["z", "z2"], observed=False):
+        for (z, z2), group in records.groupby(["z", "z2"], observed=True):
             unique_seasons = set(group["q"].unique())
             missing_seasons = seasons - unique_seasons
             if missing_seasons: