feat: log Heatmap sample filtering

[phoman14]

Changes

• Log the exact metadata column and requested sample names before Heatmap filtering.
• Log displayed filtered/total cell counts after a successful filter.
• Add focused Heatmap coverage for the custom-column filter log.

Issues

None.

PR Checklist

• This comment contains a description of changes with justifications, with any relevant issues linked.
• Write unit tests for any new features, bug fixes, or other code changes.
• Update the docs if there are any API changes (roxygen2 comments, vignettes, readme, etc.). No API changes.
• Update NEWS.md with a short description of any user-facing changes and reference the PR number.
• Run devtools::check() locally and fix all notes, warnings, and errors. Deferred: only modified-function tests were run.