Complete Julia bindings fix validation and testing

Co-authored-by: NikolajBjorner <3085284+NikolajBjorner@users.noreply.github.com>

test_julia_fix.md

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+# Julia Bindings Fix Validation
+
+## Problem
+The original issue was that when building Julia bindings on Windows with MSVC, users would get cryptic linker errors like:
+
+```
+z3jl.cpp.obj : error LNK2001: unresolved external symbol "__declspec(dllimport) struct jlcxx::CachedDatatype & __cdecl jlcxx::jlcxx_type(class std::type_index)"
+...
+..\..\..\z3jl.dll : fatal error LNK1120: 22 unresolved externals
+```
+
+This happened because:
+1. Julia's CxxWrap provides MinGW import libraries (.dll.a files)
+2. MSVC cannot link against MinGW import libraries (needs .lib files)
+3. No clear error message was provided to users
+
+## Solution Implemented
+Added detection logic in `src/api/julia/CMakeLists.txt` that:
+1. Detects when MSVC is being used on Windows
+2. Checks if the JlCxx library is a MinGW import library (.dll.a)
+3. Provides a clear error message with actionable solutions
+
+## How the Fix Works
+When `find_package(JlCxx REQUIRED)` finds a MinGW library but MSVC is being used, the build will now fail early with a helpful message:
+
+```
+Julia bindings build error: Incompatible CxxWrap library format detected.
+The found libcxxwrap_julia library (/path/to/libcxxwrap_julia.dll.a) is a MinGW import library (.dll.a), 
+but Z3 is being built with MSVC which requires .lib format.
+
+Solutions:
+1. Use MinGW/GCC instead of MSVC to build Z3
+2. Install a MSVC-compatible version of CxxWrap
+3. Disable Julia bindings with -DZ3_BUILD_JULIA_BINDINGS=OFF
+
+For more information, see: https://github.com/JuliaInterop/CxxWrap.jl#compiling-the-c-code
+```
+
+## Validation
+- ✅ Build system works correctly on Linux/GCC (no impact)
+- ✅ CMake build completes successfully 
+- ✅ All Z3 tests pass
+- ✅ Basic Z3 functionality validated
+- ✅ Julia bindings are correctly not built when not available
+- ✅ Detection logic only triggers on Windows + MSVC combination
+
+The fix provides a much better user experience by catching the incompatibility early and providing clear guidance.