fix(tool-loader): enable image output from declarative tools via data URI (#62)

* fix(tool-loader): support image output from declarative tools via data URI

Declarative tools that return image data were silently falling through
to text handling, causing models to receive raw base64 as plain text
instead of an image content block.

Detect data URIs (data:image/<type>;base64,...) in tool output and
return a proper ImageContent block. Works across all providers via
pi-ai's shared transform-messages layer; non-vision models receive a
graceful placeholder automatically.

* fix(tool-loader): address image URI review comments

- Scan lines for data URI instead of requiring stdout to start with it,
  so stray log output from scripts doesn't silently break detection
- Add mime type allowlist (jpeg, png, gif, webp) and empty data guard;
  unsupported types fall through to text handling instead of erroring silently
- Add comment explaining the magic number 5 (skipping "data:" prefix)

Author: Nivesh353

src/tool-loader.ts

@@ -118,6 +118,28 @@ function createDeclarativeTool(
 
 					// Try parsing JSON output
 					let text = stdout.trim();
+
+					// Detect data URI — works regardless of field naming in user scripts.
+					// Scan lines so stray log output before the URI doesn't break detection.
+					const dataUriLine = text.split("\n").find(
+						line => line.startsWith("data:image/") && line.includes(";base64,"),
+					);
+					if (dataUriLine) {
+						const commaIndex = dataUriLine.indexOf(",");
+						// Slice past the leading "data:" prefix (5 chars) to get "image/jpeg;base64"
+						const mimeType = dataUriLine.slice(5, commaIndex).split(";")[0];
+						const data = dataUriLine.slice(commaIndex + 1).replace(/\s/g, "");
+						const allowedMimeTypes = ["image/jpeg", "image/png", "image/gif", "image/webp"];
+						if (data && allowedMimeTypes.includes(mimeType)) {
+							resolve({
+								content: [{ type: "image", data, mimeType }],
+								details: undefined,
+							});
+							return;
+						}
+						// Fall through to text/JSON handling if data is empty or mime type unsupported
+					}
+
 					try {
 						const parsed = JSON.parse(text);
 						if (parsed.text) text = parsed.text;