Fix opcache.interned_strings_buffer being 0 in threads started later (ZTS)

[pavimus]

Prepared with an AI assistant (Claude), which also ran the reproduction and the tests on my machine; I reviewed the result. The text below is AI-generated (quoted, as asked in CONTRIBUTING.md).

Under ZTS, threads started after OPcache's startup end up with opcache.interned_strings_buffer = 0 and log a warning each; a regression from GH-23309 (PHP 8.4.26).

Since GH-23309 (PHP 8.4.26) OnUpdateInternedStringsBuffer() rejects any change once OPcache is
set up. Under ZTS every thread started after OPcache's startup re-applies the INI entries with
ZEND_INI_STAGE_STARTUP into its own zeroed zend_accel_globals; the handler rejects that, logs a
warning for every new thread ("opcache.interned_strings_buffer cannot be changed when OPcache is
already set up.", seen once per thread in Apache error logs and FrankenPHP) and leaves the
directive at 0 in that thread (opcache_get_configuration() reports 0).

Accept the value at ZEND_INI_STAGE_STARTUP (the one SHM was set up with); changes at later
stages, like FPM pools (ZEND_INI_STAGE_ACTIVATE), are still rejected as intended by GH-23309.
Verified from a FrankenPHP worker thread: 0 -> 8, no warnings.

No test: the problem needs a thread started after startup (FrankenPHP, Apache threaded MPM).

🤖 Generated with Claude Code

[pavimus]

Prepared with an AI assistant (Claude); the text below is AI-generated (quoted, as asked in CONTRIBUTING.md).

Related: the same problem for opcache.memory_consumption on PHP 8.5 is GH-22216, with the identical fix in #22225 by @dkkoma. Both can go in together; when merging this one up into PHP-8.5 the two handlers end up with the same check.