fix: include offending values in mapping_section assertion error (#1367)

[wangzhengdna-lang]

Issue

Closes #1367

Description

The assertion in .column_mapping_section() (inst/shiny/modules/tab_data/data_mapping.R) threw a generic message when mapping_df contained zero or multiple mapping_section values, giving no clue about the actual content. The error now reports the count and the offending values, e.g.:

mapping_df must contain exactly one unique mapping_section value, but found 2: PK, PD

Definition of Done

• Error message reports the number and the actual mapping_section values found

How to test

Defensive guard message change — exempt from unit tests per project convention. Verified manually by calling .column_mapping_section() with a two-section data frame, which now produces the message shown above.

Contributor checklist

• Code passes lintr checks
• Code passes all unit tests
• New logic covered by unit tests
• New logic is documented
• App or package changes are reflected in NEWS
• Package version is incremented
• R script works with the new implementation (if applicable)
• Settings upload works with the new implementation (if applicable)
• If any .scss change was done, run data-raw/compile_css.R
• If a package dependency was added/changed, run data-raw/test_suggests_hidden.R

Notes to reviewer

One-line error message improvement; full test suite 2005 PASS / 0 FAIL locally, spellcheck clean.