Harden type handling for PHP 8.5 compatibility

[rhoerr]

Summary

PHP 8.5 promotes several previously-deprecated coercions to errors — notably casting non-scalar values to string. This PR hardens type handling in places where mixed / untyped values flow into typed APIs, so the module is forward-compatible.

• ViewModel/SpeculationRules::getConfigValue — guard the mixed scopeConfig->getValue() result with is_scalar before casting to string. Previously a non-scalar config value (rare, but possible via custom backends) would fatal in PHP 8.5.
• ViewModel/SpeculationRules::getSpeculationRulesJson — handle SerializerInterface::serialize() returning false. The interface return is string|false; the method signature promised string.
• ViewModel/ViewTransitions::getConfigValue — same scalar guard as above.
• Plugin/Framework/App/Response/Http — narrow getHeader() result with instanceof checks. Laminas Headers::get() returns false|HeaderInterface|ArrayIterator; the previous if (!$header) only ruled out false, leaving ArrayIterator callable as if it were a HeaderInterface. removeDirective() lives on CacheControl, not HeaderInterface, so the second narrowing uses that concrete class. Also dropped a redundant second getHeader() call in afterSetNoCacheHeaders.
• Plugin/Framework/App/Response/Http::getConfig — same scalar guard before casting to string.
• Setup/Patch/Data/UpdateSpeculationRulesConfigPathPatch::apply — return $this to match PatchInterface docblock (@return $this). Not a PHP 8.5 issue per se, but caught by phpstan and trivial to align.

Verification

• phpstan level 9 against the module — clean (was 8 errors before)
• phpunit against app/code/MageOS/ThemeOptimization/phpunit.xml.dist — 41/41 pass

Test plan

• CI green
• No regression on bfcache header behavior in built-in FPC mode
• Speculation rules JSON still emitted for stores with non-default mode/eagerness/exclude_* config
• setup:upgrade still applies UpdateSpeculationRulesConfigPathPatch cleanly

🤖 Generated with Claude Code