Fix incomplete error handling in MediaStream::Configure

[Copilot]

When gst_element_link or gst_element_link_many failed after elements were added to the pipeline bin, the code returned early without removing them. This left orphaned elements in the bin with null/uninitialized member pointers.

Changes

• Added cleanup on link failures: Both error paths now call gst_bin_remove_many() to remove elements from the bin before returning, allowing RAII wrappers to properly destroy them
• Extracted cleanup logic: Introduced cleanup_on_error lambda to eliminate code duplication across error paths

// Helper lambda to clean up elements from bin on error
auto cleanup_on_error = [&]() {
    gst_bin_remove_many(
        GST_BIN(pipeline),
        appsrcLocal.get(),
        decodebinLocal.get(),
        convLocal.get(),
        postLocal.get(),
        sinkLocal.get(),
        nullptr);
};

// --- Link appsrc -> decodebin ---
if (!gst_element_link(appsrcLocal.get(), decodebinLocal.get()))
{
    GST_ERROR("gst_element_link(appsrc->decodebin) failed");
    cleanup_on_error();
    return;
}

This ensures the bin releases its references, preventing memory leaks and maintaining consistent state between bin ownership and member pointers.

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