ref: make SENTRY_HANDLER_STACK_SIZE overridable via env var

[jpnurmi]

Allow overriding the compile-time SENTRY_HANDLER_STACK_SIZE default at runtime via environment variable. The stack-overflow integration tests previously parametrized SENTRY_HANDLER_STACK_SIZE as a CMake option, forcing 5 extra configure+build cycles on Windows (crashpad x2, inproc x1, breakpad x2). Switching to a runtime env var lets every parametrization reuse the cached build, saving a minute or two on Windows.

	Before (s)	After (s)
test_crashpad_dumping_stack_overflow (default/16/32)	1.56 / 61.44 / 62.19	1.57 / 1.67 / 1.58
test_inproc_stack_overflow_stdout (default/32)	1.61 / 20.33	1.86 / 1.95
test_breakpad_stack_overflow_stdout (default/16/32)	0.12 / 21.20 / 21.57	0.13 / 0.11 / 0.12

#skip-changelog

[supervacuus]

Just a quick heads-up: the reason this is a build-time variable is its use inside DllMain, where we must ensure that we don't depend on anything but kernel32.dll. That it works in CI now is nice, but having both getenv() and strtod() usage (both CRT functions) in DllMain gives me nightmares, because it creates a latent instability only waiting to raise issues... and that would be again a change in the production code for a win in testing.

If we truly want to improve the test build cache usage, let's not parametrize the build-time parameter in the test; instead, supply the stack size as a CLI parameter in the example and set everything there at runtime. This eliminates multiple configs that miss the cache without destabilizing auto-init by introducing dependencies beyond kernel32.dll. Another option would be an envreader/parser that depends solely on kernel32.dll.

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