[Flutter GPU] DeviceBuffer.flush() with defaults flushes the whole buffer - #194162

DeviceBuffer.flush() defaults lengthInBytes to -1, documented as "the whole buffer", but InternalFlutterGpu_DeviceBuffer_Flush passed it straight into an impeller::Range of size_t, so flush() became Flush(Range{0, SIZE_MAX}). On OpenGL ES, DeviceBufferGLES keeps that as its dirty range and uploads it with glBufferSubData(..., SIZE_MAX, ...), which fails with GL_INVALID_VALUE: the data never reaches the GPU, and a debug engine aborts on the GL error check. Metal gets a didModifyRange: past the end of the buffer. Vulkan only worked because SIZE_MAX happens to equal VK_WHOLE_SIZE.

The native entry point now treats a negative length as "to the end of the buffer" and returns false for a range outside the buffer, with the same offset bound the Dart side checks. The Dart binding was declared as returning Void, so that result was dropped; it is Bool now, and flush throws when the engine refuses, the way overwrite does. -1 is normalized in one place, on the native side, so the entry point can't build an oversized range whoever calls it.

device_buffer_unittests.cc is new and uses a MockDeviceBuffer with MOCK_METHOD(Flush) to check the Range that reaches Flush.

Where it was verified. macOS arm64, host_debug_unopt_arm64, rebased on current main. Without the change three of the four C++ tests fail (Flush(Range{0, 0xFFFFFFFFFFFFFFFF})), and a new gpu_test.dart test that draws a triangle from a buffer flushed with defaults aborts under flutter_tester_opengles with Fatal GL Error GL_INVALID_VALUE(1281) encountered on call to glBufferSubData; Metal and Vulkan (SwiftShader) pass it. With the change gpu_unittests --gtest_filter='FlutterGpuDeviceBufferTest.*:FlutterGpuRenderPassTest.*:FlutterGpuRenderPipelineTest.*:FlutterGpuShaderLibraryTest.*' is 17/17 and gpu_test.dart passes on all three backends. FlutterGpuCommandBufferTest.* is left out of the filter because it aborts in unopt builds on main as well (#194149).

Fixes #194152

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