fix(coding-agent): process BMP images from disk

closes #6047

packages/coding-agent/test/image-process.test.ts

@@ -0,0 +1,53 @@
+import { describe, expect, it } from "vitest";
+import { processImage } from "../src/utils/image-process.ts";
+import { detectSupportedImageMimeType } from "../src/utils/mime.ts";
+
+function createTinyBmp1x1Red24bpp(): Buffer {
+	const buffer = Buffer.alloc(58);
+	buffer.write("BM", 0, "ascii");
+	buffer.writeUInt32LE(buffer.length, 2);
+	buffer.writeUInt32LE(54, 10);
+	buffer.writeUInt32LE(40, 14);
+	buffer.writeInt32LE(1, 18);
+	buffer.writeInt32LE(1, 22);
+	buffer.writeUInt16LE(1, 26);
+	buffer.writeUInt16LE(24, 28);
+	buffer.writeUInt32LE(0, 30);
+	buffer.writeUInt32LE(4, 34);
+	buffer[56] = 0xff;
+	return buffer;
+}
+
+function expectPngMagic(base64Data: string): void {
+	const buffer = Buffer.from(base64Data, "base64");
+	expect(buffer[0]).toBe(0x89);
+	expect(buffer[1]).toBe(0x50);
+	expect(buffer[2]).toBe(0x4e);
+	expect(buffer[3]).toBe(0x47);
+}
+
+describe("image processing pipeline", () => {
+	it("detects BMP files from magic bytes", () => {
+		expect(detectSupportedImageMimeType(createTinyBmp1x1Red24bpp())).toBe("image/bmp");
+	});
+
+	it("converts BMP files to PNG attachments when auto-resize is disabled", async () => {
+		const result = await processImage(createTinyBmp1x1Red24bpp(), "image/bmp", { autoResizeImages: false });
+
+		expect(result.ok).toBe(true);
+		if (!result.ok) return;
+		expect(result.mimeType).toBe("image/png");
+		expect(result.hints).toContain("[Image converted from image/bmp to image/png.]");
+		expectPngMagic(result.data);
+	});
+
+	it("converts BMP files before auto-resizing", async () => {
+		const result = await processImage(createTinyBmp1x1Red24bpp(), "image/bmp");
+
+		expect(result.ok).toBe(true);
+		if (!result.ok) return;
+		expect(result.mimeType).toBe("image/png");
+		expect(result.hints).toContain("[Image converted from image/bmp to image/png.]");
+		expectPngMagic(result.data);
+	});
+});