Linux Crashpad fix (#890)

- **Replace crashpad static-libc++ patch file with `io.replace()` in `on_install`** — The old `.patch` file was fragile (trailing-whitespace stripping on Windows would silently break it). Using `io.replace()` in the xmake build script is more robust and easier to maintain.
- **Skip sentry-native `on_test` link check on Linux** — The link test requires `-lc++abi` when building with the UE clang toolchain but adding it unconditionally breaks GCC/libstdc++ builds. The zenserver build itself validates that the library is usable.
- **Add `crashpad-test.sh`** — A test script that launches a release zenserver, waits for the health endpoint, then verifies that `crashpad_handler` is running, no `sentry_init` failure was logged, and the handler has no dynamic `libc++.so.1` dependency.
- **Add Crashpad Check step to Linux release CI** — Runs `crashpad-test.sh` in the `validate` workflow for release builds to catch crashpad regressions before merge.

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