[3.0] Parse memory settings that carry no unit designator

[albertlast]

Note

This change was produced by an LLM. The code, the commit message and this
description were all written by Claude (Anthropic), driven by @albertlast. It has
not yet had human code review.

Everything stated below was verified by actually running it, rather than only
reasoned about. Even so, please review it as untrusted work: the diagnosis may be
right while the fix is not what SMF would prefer stylistically or architecturally.

Description

Sapi::memoryReturnBytes() removes the last character of the value before parsing
the number, on the assumption that it is always a unit designator:

$num = \intval(substr($val, 0, \strlen($val) - 1));
$last = strtolower(substr($val, -1));

PHP's shorthand notation is optional, so a setting may be a plain byte count. When it
is, the last digit is eaten: '128' reads as 12, and '2097152' reads as 209715.

Sources/Graphics/Image.php does exactly that. It works out how much memory an image
needs and passes the raw byte count with no designator:

return Sapi::setMemoryLimit((string) $needed_memory, true);

so image resizing asks for a tenth of what it just calculated.

The other value carrying no designator is -1, which is how PHP spells "no limit".
substr('-1', 0, 1) is '-', and intval('-') is 0, so an unlimited server reports
zero bytes available. setMemoryLimit() then finds the current limit smaller than
anything at all and imposes one.

This strips the last character only when it is a designator PHP actually accepts, and
reports "no limit" as PHP_INT_MAX so that the four call sites comparing it against an
amount they need do not each have to special case it. The dead is_integer() check at
the top went with it; the parameter is typed string, so it never fired.

How this was verified

PHP 8.4.23. Same script before and after the change.

Parsing:

value	before	after
'256M'	268435456	268435456
'1G'	1073741824	1073741824
'512K'	524288	524288
'128'	12	128
'2097152'	209715	2097152
'-1'	0	PHP_INT_MAX
' 64M '	67108864	67108864

Then through the caller, with memory_limit set to -1:

before:  setMemoryLimit('128M') -> memory_limit is now 128M
after:   setMemoryLimit('128M') -> memory_limit is still -1

Nothing that already worked changes: every value with a designator parses to the same
number as before.

php -l and PHP-CS-Fixer clean.

Relationship to other PRs

Sources/Sapi.php is not touched by any other open PR.

Found while writing unit tests for the stateless parts of the codebase (#9326). That
branch carries the regression tests for this fix, and reverting this file alone fails
exactly those tests. This PR is deliberately independent of it, so it can be merged on
its own.

Issues References (Fixes|Related|Closes)

1. No existing issue found for this.
2. Related: [3.0][Testing] Add a PHPUnit suite for the parts that need no database #9326

[Sesquipedalian]

We should check whether this is also an issue in SMF 2.1 and fix it if so.

EDIT: Yes, it is. I'll submit a PR for that shortly.