gh-153838: Skip non-regular source files in the heatmap exporter (#153839)

* gh-153838: Skip non-regular and oversized files in heatmap exporter

* Drop the size cap, keep the is_file() guard only

* gh-153838: Verify non-regular sources are never opened

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Co-authored-by: Pablo Galindo Salgado <Pablogsal@gmail.com>