BOT: Fix #625: Implement get_unique_values() function

[nikosbosse]

Summary

• Closes Implement a get_unique_values() function #625
• Adds a new exported get_unique_values() function that counts the number of unique values in each forecast unit column of a forecast object
• Accepts an optional by argument for grouping (e.g., by = "model" to see unique value counts per model)
• Follows the same patterns as get_forecast_counts(): validates input via clean_forecast(), uses get_forecast_unit() to identify relevant columns, returns a plain data.table

Root cause

No diagnostic function existed to quickly summarize the scope of a forecast dataset (how many locations, models, dates, etc.).

What the fix does

• Creates R/get-unique-values.R with the exported get_unique_values() function
• The function takes a forecast object, identifies forecast unit columns via get_forecast_unit(), and returns a data.table with columns "column" and "N_unique"
• Supports optional by argument for grouped summaries
• Adds N_unique to globalVariables in R/z-globalVariables.R

Test coverage

• 6 test cases in tests/testthat/test-get-unique-values.R (26 expectations total):
  • Works with quantile forecasts and verifies exact unique counts
  • Works across different forecast types (binary, sample)
  • Returns correct structure with minimal point forecast data
  • Supports by argument for grouped output
  • Errors on non-forecast input
  • Handles data with NAs correctly
• Full test suite: 704 tests pass, 0 failures
• R CMD check: 0 errors, 0 warnings, 2 notes (pre-existing)

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.85%. Comparing base (ac0c01a) to head (1c63583).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1095      +/-   ##
==========================================
+ Coverage   97.83%   97.85%   +0.02%     
==========================================
  Files          35       36       +1     
  Lines        1845     1865      +20     
==========================================
+ Hits         1805     1825      +20     
  Misses         40       40              

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[nikosbosse]

CLAUDE: Clean implementation of get_unique_values() that follows established patterns (get_forecast_counts). Function correctly uses clean_forecast + get_forecast_unit, supports optional by argument for grouping, and has comprehensive test coverage (6 tests, 26 expectations). All changes are well-scoped to the issue. Minor nits: out_list variable could be simplified, and Test 4 assertions could be more precise — neither blocking. Verdict: approve.