BOT: Fix #406: Rename get_type() to get_vector_type() and add convenience wrappers

[nikosbosse]

Summary

• Renames the internal get_type() function to get_vector_type() and exports it, giving users access to the vector/matrix type detection utility
• Adds get_observed_type() wrapper that extracts the observed column from a forecast object and returns its type
• Adds get_predicted_type() wrapper that extracts the predicted column from a forecast object and returns its type
• Updates all 3 internal call sites in R/metrics-sample.R to use the new name

Root cause: get_type() was a useful utility function marked as internal (@keywords internal), with no user-facing way to determine the type of observed or predicted values in a forecast object.

Closes #406

Test plan

• Existing get_type() tests updated to use get_vector_type() name (vectors, matrices, NA handling, consistency with old prediction_type())
• New test: get_vector_type() is exported and accessible via scoringutils::
• New test: get_observed_type() returns correct type for sample, binary, and discrete forecasts
• New test: get_observed_type() errors on non-forecast objects
• New test: get_predicted_type() returns correct type for sample, binary, and quantile forecasts
• New test: get_predicted_type() errors on non-forecast objects
• Full test suite passes (690 tests, 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 97.83%. Comparing base (ac0c01a) to head (e3c73f2).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1099   +/-   ##
=======================================
  Coverage   97.83%   97.83%           
=======================================
  Files          35       35           
  Lines        1845     1849    +4     
=======================================
+ Hits         1805     1809    +4     
  Misses         40       40           

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[nikosbosse]

CLAUDE: Clean, well-scoped rename of internal get_type() to exported get_vector_type() plus two thin convenience wrappers (get_observed_type, get_predicted_type). No logic changes to the core function body. All 3 internal call sites in metrics-sample.R correctly updated. No remaining references to old name. Both wrappers properly validate input via assert_forecast(). Comprehensive test coverage with 12 test blocks covering all specs. R CMD check clean. Verdict: APPROVE.