refactor: cgi strings

[AlliBalliBaba]

This PR does a bit of cleanup with how Cgi strings are registered by moving the zend_string keys to a struct on the C side. This also has some performance benefits, mainly by not having to do map access and correctly sizing the $_SERVER array.

Server registration goes from around 5.5% of request time to around 3.5%.

Draft as waiting for #2058 first.

[Copilot]

Pull request overview

Refactors how CGI/$_SERVER string keys are managed by moving hard-coded zend_string keys into a C-side interned-strings struct, reducing Go-side map lookups/allocations and enabling more efficient $_SERVER sizing during registration.

Changes:

• Introduces C-side interned string registry (frankenphp_strings) and a frankenphp_server_vars struct to bulk-register known $_SERVER variables with pre-sized hashtable capacity.
• Simplifies Go-side CGI/server-variable registration to rely on C-side known keys, keeping only the common-header cache map in Go.
• Minor header handling refactor in Go to pass sapi_header_struct directly.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
phpmainthread.go	Moves common header zend_string cache to a package-global map and initializes it once.
frankenphp.h	Defines frankenphp_server_vars and frankenphp_strings interned-string registry; updates exported APIs.
frankenphp.go	Adjusts header parsing helper to accept sapi_header_struct.
frankenphp.c	Implements interned-string initialization, server var bulk registration, and updates request variable registration flow.
env.go	Captures OS env count for $_SERVER pre-sizing; switches to newPersistentZendString.
cgi.go	Reworks known CGI var registration to use C-side interned keys/values; adds tlsProtocol and helper wrappers.

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