Merge pull request #28981 from Srujan-rai:fix/opengl-extensions-memory-leak

core(opengl): fix memory leak in OpenCL extensions gathering #28981

## Problem

Fixes #28980

In `modules/core/src/opengl.cpp`, inside `initializeContextFromGL()`, a `char[]` buffer is allocated to query OpenCL device extensions:

```cpp
extensions = new char[extensionSize];
status = clGetDeviceInfo(..., extensions, &extensionSize);

if (status != CL_SUCCESS)
    continue;  // leaks `extensions`
```

When `clGetDeviceInfo()` fails, `continue` skips the corresponding `delete[]` on the success path, causing the allocated buffer to leak.

Additionally, the `catch (...)` block also bypasses cleanup, so any thrown exception leaks the buffer as well.

## Fix

Replace the raw `char*` allocation with `std::unique_ptr<char[]>`.

This ensures the buffer is automatically released on all exit paths, including:

- normal execution
- early `continue`
- exception handling paths

## Checklist

- [x] I agree to contribute to the project under the Apache 2 License.
- [x] To the best of my knowledge, the proposed patch is not based on code under GPL or another license incompatible with OpenCV.
- [x] The PR is proposed to the proper branch (`4.x`).
- [x] There is a reference to the original bug report and related work, if applicable.
- [x] Accuracy/performance tests and test data in `opencv_extra` are included, if applicable.
- [x] The feature/change is documented and sample code can be built with the project CMake configuration, if applicable.

modules/core/src/opengl.cpp

@@ -1682,27 +1682,12 @@ Context& initializeContextFromGL()
 
                 if(extensionSize > 0)
                 {
-                    char* extensions = nullptr;
-
-                    try {
-                        extensions = new char[extensionSize];
-
-                        status = clGetDeviceInfo(devices[j], CL_DEVICE_EXTENSIONS, extensionSize, extensions, &extensionSize);
-                        if (status != CL_SUCCESS)
-                            continue;
-                    } catch(...) {
-                        CV_Error(cv::Error::OpenCLInitError, "OpenCL: Exception thrown during device extensions gathering");
-                    }
-
-                    std::string devString;
-
-                    if(extensions != nullptr) {
-                        devString = extensions;
-                        delete[] extensions;
-                    }
-                    else {
-                        CV_Error(cv::Error::OpenCLInitError, "OpenCL: Unexpected error during device extensions gathering");
-                    }
+                    std::string devString(extensionSize, '\0');
+                    status = clGetDeviceInfo(devices[j], CL_DEVICE_EXTENSIONS, devString.size(), &devString[0], &extensionSize);
+                    if (status != CL_SUCCESS)
+                        continue;
+                    if (extensionSize > 0 && devString.size() >= extensionSize)
+                        devString.resize(extensionSize - 1);
 
                     size_t oldPos = 0;
                     size_t spacePos = devString.find(' ', oldPos); // extensions string is space delimited