`naturalsize()` raises `ValueError` for custom format strings with text

Environment

• humanize 4.16.0 and current main at 42b4a1db
• Python 3.11.15

Reproduction

import humanize

print(humanize.naturalsize(999_999, gnu=True, format="Size: %.1f"))

humanize 4.15.0 returns:

Size: 976.6K

humanize 4.16.0 and current main raise:

ValueError: could not convert string to float: 'Size: 976.6'

Expected behavior

The documented custom formatter should continue to accept text around the numeric conversion.
Since this behavior worked in 4.15.0, preserving it would avoid a backward compatibility regression.

Cause

PR #329 added unit rollover handling. The new condition formats the mantissa with the
user-provided format and passes the entire result to float():

float(format % (abs_bytes / (base**exp)))

This works for a plain numeric format such as "%.1f", but not for a general formatter
such as "Size: %.1f".

Suggested fix

It might be preferable not to require the complete user-formatted result to be parseable as a float. One
backward-compatible option is to apply rollover only when the formatted result is a
plain numeric string, and otherwise preserve the pre-4.16 behavior. A more complete
solution could derive the rounded numeric mantissa independently from surrounding text.

It might also be worth adding a regression test covering both a prefix and a suffix.

How this was found

This was found by change-directed differential testing: reviewing the recent rollover change, mapping it to the public naturalsize() API, and constructing a documented custom formatter that reaches the newly added float(format % value) expression.

Related: #329, which fixes rollover but does not discuss custom format strings.

Thanks for taking the time to review this report! I appreciate all the work that goes into maintaining this project.

Partite dal punto di ingresso naturalsize() e analizzate la gestione del riporto dell’unità introdotta dalla PR #329, in particolare il percorso del formato personalizzato descritto nel report. Aggiungete una copertura di regressione per le stringhe di formato con prefissi e suffissi, quindi eseguite la suite di test pertinente; il lavoro è completo quando questi formati mantengono il comportamento precedente alla versione 4.16, mentre il riporto continua a funzionare per i formati numerici semplici.