ci: register schemagen regression tests with ctest

Remove the standalone "Run schemagen tests" workflow step and instead
add the big-schema regression test to contrib/schemagen/CMakeLists.txt
so ctest picks it up alongside schemagen_example. Update README to
document both `ctest` and the convenience `./run_tests.sh`.

Closes #3

contrib/schemagen/README.md

@@ -76,15 +76,25 @@ union `type` lists other than `["T", "null"]`, non-string enums, and remote
 
 ## Testing
 
-After building weaseljson (e.g. `cmake -S . -B build && make -C build`), run:
+The schemagen tests are registered with CTest and run as part of the default
+`ctest` invocation from the build directory:
+
+```sh
+cmake -S . -B build
+make -C build -j "$(nproc)"
+cd build
+ctest --output-on-failure
+```
+
+For local development you can also use the convenience script:
 
 ```sh
 cd contrib/schemagen
 ./run_tests.sh
 ```
 
-This regenerates the example parser (`gen.h`) and a regression parser with 40
-required properties (`big.h`), compiles `test_gen.cpp` and `test_big.cpp`, and
-runs both. `test_big.cpp` specifically covers issue #3: it checks that a
-40-property object accepts all fields, rejects a missing field at index 32, and
-rejects duplicate keys around the 32-bit boundary.
+Both regenerate the example parser (`gen.h`) and a regression parser with 40
+required properties (`big.h`), compile `test_gen.cpp` and `test_big.cpp`, and run
+them. `test_big.cpp` specifically covers issue #3: it checks that a 40-property
+object accepts all fields, rejects a missing field at index 32, and rejects
+duplicate keys around the 32-bit boundary.