Fix -Wtypedef-redefinition errors in public headers for C99 compatibility

[otegami]

Summary

Remove duplicate typedef definitions in zl_graph_api.h that cause compilation errors when external projects include OpenZL headers with C99 mode and -Werror -Wtypedef-redefinition.

Before this fix, compiling with C99 produced:

zl_graph_api.h:48:27: error: redefinition of typedef 'ZL_Graph' is a C11 feature [-Werror,-Wtypedef-redefinition]
   48 | typedef struct ZL_Graph_s ZL_Graph;
      |                           ^
zl_opaque_types.h:61:27: note: previous definition is here
   61 | typedef struct ZL_Graph_s ZL_Graph;
      |                           ^
zl_graph_api.h:49:26: error: redefinition of typedef 'ZL_Edge' is a C11 feature [-Werror,-Wtypedef-redefinition]
   49 | typedef struct ZL_Edge_s ZL_Edge;
      |                          ^
zl_opaque_types.h:62:26: note: previous definition is here
   62 | typedef struct ZL_Edge_s ZL_Edge;
      |                          ^
zl_graph_api.h:161:37: error: redefinition of typedef 'ZL_RuntimeGraphParameters' is a C11 feature [-Werror,-Wtypedef-redefinition]
  161 | typedef struct ZL_GraphParameters_s ZL_RuntimeGraphParameters;
      |                                     ^
zl_introspection.h:12:37: note: previous definition is here
   12 | typedef struct ZL_GraphParameters_s ZL_RuntimeGraphParameters;
      |                                     ^
3 errors generated.

Type of Change

Bug fix (non-breaking change which fixes an issue)

Test Plan

Prepare: Build OpenZL and install

cmake \
  -S . \
  -B ../openzl.build \
  -DCMAKE_C_COMPILER=clang \
  -DCMAKE_CXX_COMPILER=clang++ \
  -DOPENZL_BUILD_TESTS=ON \
  -DCMAKE_INSTALL_PREFIX=/tmp/local && \
cmake --build ../openzl.build &&\
cmake --install ../openzl.build

Verify C99 compilation works with installed headers

Confirm that including OpenZL headers compiles without errors in C99 mode with -Werror -Wtypedef-redefinition.

echo '#include "openzl/zl_graph_api.h"
int main() { return 0; }' > /tmp/test.c

clang -std=c99 -Werror -Wtypedef-redefinition \
      -I/tmp/local/include \
      -c /tmp/test.c -o /tmp/test.o

Run all tests

Verify no regressions: ctest (1202 tests passed)

Test Configuration

• Compiler: Clang 18.1.8
• Build type: none
• Platform(s): Linux (Ubuntu noble)