Implement proper zval to string conversion in av_value_to_string

[matapatos]

Summary

• Implements proper, type-aware conversion of any PHP zval to a zend_string in av_value_to_string, replacing the placeholder // TODO implementation that delegated to zval_get_string.
• Conversion handles every zval type for clear error messages:
  • null → null
  • bool → true / false (previously 1 / empty string)
  • int / double → numeric strings via zend_long_to_str / zend_double_to_str
  • string → single-quoted value
  • array → array
  • object → __toString() result when Stringable, otherwise the class name
  • resource → resource type name (or resource)
  • references are dereferenced before inspection
• Adds Ceedling C unit tests (tests/c/test_value_to_string.c) covering all of the above plus the Stringable/__toString and resource-unknown-type fallback paths.

Testability refactor

• Moved av_value_to_string into its own translation unit (src/helpers/av_value_to_string.{c,h}) so tests link only that minimal file instead of the full av_error_messages.c dependency graph. Registered in config.m4.
• Routed all Zend internals the function touches through mockable wrappers in av_wrappers.{c,h}: av_string_concat3, av_string_copy, av_long_to_str, av_double_to_str, av_is_stringable, av_instanceof_function, av_rsrc_list_get_rsrc_type, av_call_tostring, av_zval_ptr_dtor.
• Bug fix exposed by the tests: the Stringable branch compared the return of zend_call_method_with_0_params (a zval*) to SUCCESS, which is always false — so __toString was a dead path and objects always fell back to the class name. av_call_tostring now reports success only when __toString returns a string.
• Taught CMock (project.yml) to treat zend_class_entry/zend_object/zend_resource as void so mock pointer args compare by value instead of trying to sizeof an incomplete struct.

Verification

• ceedling test:all → 114/114 pass (89 existing + 25 new for av_value_to_string).
• Full extension build (phpize && ./configure --enable-attributes-validation && make) → Build complete, zero warnings in changed files (PHP 8.4).
• cc -Wall -Wextra -fsyntax-only src/helpers/av_value_to_string.c → clean.

Note: PHP/Ruby toolchains were not preinstalled in the environment; I installed php-dev (PHP 8.4), ruby, and ceedling to compile-verify. The repo targets PHP 8.2+; the Zend APIs used are stable across PHP 8.2–8.4. The existing PHP integration tests do not exercise the {value} placeholder, so current expectations (e.g. The value must be integer or float.) are unaffected.