* Fix observer cache poisoning in FrankenPHP worker mode (#63)

Three coupled changes are needed for line breakpoints to fire on
functions called by trigger requests after non-trigger requests have run
in the same worker:

1. xdebug_observer_init: always return real handlers. The Zend engine
   caches the result per zend_function the first time the function is
   observed, so returning {NULL, NULL} when observer_active=false
   permanently blacklists that function — even after a debugger
   connects in a later request that reuses the cached op_array. The
   handlers themselves already fast-path on \!observer_active, so the
   no-debug overhead remains a single load+branch.

2. xdebug_frankenphp_sapi_activate: re-arm observer_active per request.
   FrankenPHP runs PHP_RINIT only once at worker startup; per-request
   only sapi_activate fires. Without this, a worker that started with
   no IDE leaves observer_active=false for the rest of its life, so
   xdebug_execute_begin fast-paths out, no fse is pushed, and
   xdebug_debugger_statement_call bails on its empty-stack check.

3. xdebug_frankenphp_minit: force ZEND_COMPILE_EXTENDED_STMT and
   disable the opcache optimizer at MINIT. The normal RINIT path skips
   this when no IDE is connected at startup, but in the worker flow
   user files are compiled later — by trigger requests that need
   EXT_STMT opcodes already present. Without this, breakpoint_set sees
   an empty "set of executable lines" for the target file.

The `tests/frankenphp/app` workload (`workload()` defined in `lib.php`,
called from `index.php`) plus `dbgp_listener.py` reproduce the failure
deterministically: 200 no-trigger requests followed by 5 trigger
requests previously yielded 0/5 breakpoint hits; with the fix all 5
hit lib.php:4.

* Last changes to finish the task

* Changes after claude review

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Co-authored-by: Roman Pronskiy <roman@pronskiy.com>