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.

IMPLEMENTATION_PLAN.md

@@ -90,7 +90,7 @@ python -m reflex compile
 ## Phase 3: Test Full Pipeline
 
 ### 3.1 Test Refresh with Real Data
-- [ ] Run `python -m cli.refresh_pathways --chart-type all` with Snowflake
+- [~] Run `python -m cli.refresh_pathways --chart-type all` with Snowflake
 - [ ] Verify pathway_nodes table has both chart_type values:
   - `SELECT chart_type, COUNT(*) FROM pathway_nodes GROUP BY chart_type`
 - [ ] Verify indication hierarchy: Trust → Search_Term → Drug → Pathway