fix: resolve Snowflake column casing and UPID mapping issues (Task 3.1)

Three issues identified and fixed during Task 3.1 testing:

1. Snowflake column name casing:
   - Unquoted columns in Snowflake are returned as UPPERCASE
   - Fixed by aliasing columns with quoted names: AS "Search_Term"
   - Now correctly populates 139 unique Search_Terms (was 0)

2. Duplicate UPID index error:
   - indication_df_for_chart could have duplicate UPIDs
   - Added drop_duplicates(subset=['UPID']) before set_index()
   - Keeps first occurrence (DIAGNOSIS over FALLBACK)

3. Missing UPIDs in indication lookup:
   - Old code: built indication_df from unique PseudoNHSNoLinked only
   - Problem: patients with multiple UPIDs (multi-provider) were missing
   - Fixed: now builds indication_df from ALL unique UPIDs in df
   - Also handles NaN values in Directory column safely

Validation results from test run:
- 36,628 patients queried
- 34,006 (92.8%) had GP diagnosis matches
- 139 unique Search_Terms found
- Top 5: drug misuse (8602), influenza (6239), diabetes (2476)

Still to verify: full pathway processing after these fixes.

data_processing/diagnosis_lookup.py

@@ -1348,12 +1348,13 @@ def get_patient_indication_groups(
 
         # Build the full query with cluster CTE
         # This finds the most recent matching diagnosis for each patient
+        # Note: Column names must be aliased to ensure consistent casing in results
         query = f"""
 {CLUSTER_MAPPING_SQL}
 SELECT
-    pc."PatientPseudonym",
-    aic.Search_Term,
-    pc."EventDateTime"
+    pc."PatientPseudonym" AS "PatientPseudonym",
+    aic.Search_Term AS "Search_Term",
+    pc."EventDateTime" AS "EventDateTime"
 FROM DATA_HUB.PHM."PrimaryCareClinicalCoding" pc
 INNER JOIN AllIndicationCodes aic
     ON pc."SNOMEDCode" = aic.SNOMEDCode