fix: handle stream errors in runAction to prevent circular JSON crash

[huangjeff5]

If the runtime returns an error stream, that was causing tools server to crash with "Converting circular structure to JSON" crash error. This makes sense because you can't call JSON.parse on a stream, it's not serializable due to circular references. This PR ensures that the error passed to httpErrorHandler is serializable.

[gemini-code-assist]

Summary of Changes

Hello @huangjeff5, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request resolves a server stability issue where the genkit-tools server would crash when encountering error responses delivered as streams, due to JSON.stringify failing on circular references inherent in stream objects. The changes introduce a dedicated mechanism to intercept and properly serialize stream-based error data before it's processed by the general HTTP error handler, thus preventing crashes and enhancing the robustness of error reporting.

Highlights

• Stream Error Handling: Introduced a new handleStreamError method to specifically process errors that occur during stream-based API calls, ensuring proper serialization of error data.
• Crash Prevention: Addressed and fixed a critical issue where the tools server would crash with a 'Converting circular structure to JSON' error when a runtime returned an error as a stream.
• Error Data Serialization: Implemented logic to read error stream content into a string, attempt JSON parsing, and replace the original stream object with a serializable error object, preventing circular reference issues.

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[gemini-code-assist]

Code Review

This pull request effectively addresses a crash caused by attempting to serialize a stream object in error responses. The introduction of handleStreamError to process error streams before they are passed to the error handler is a solid approach. The change of httpErrorHandler's return type to never is also a good improvement for type safety. I've identified one potential issue in the new streamToString helper function that could lead to data corruption, and I've provided a suggestion to fix it.

[gemini-code-assist]

The current implementation of streamToString can lead to corruption of multi-byte characters if they are split across data chunks. Calling chunk.toString() on partial character data will result in replacement characters (``).

To fix this, you should collect all Buffer chunks in an array, concatenate them into a single Buffer when the stream ends, and then convert that final buffer to a string. This ensures that multi-byte characters are decoded correctly.

Suggested change

	private streamToString(stream: any): Promise<string> {
	return new Promise((resolve, reject) => {
	let buffer = '';
	stream.on('data', (chunk: Buffer) => {
	buffer += chunk.toString();
	});
	stream.on('end', () => {
	resolve(buffer);
	});
	stream.on('error', (err: Error) => {
	reject(err);
	});
	});
	}
	private streamToString(stream: any): Promise<string> {
	return new Promise((resolve, reject) => {
	const chunks: Buffer[] = [];
	stream.on('data', (chunk: Buffer) => {
	chunks.push(chunk);
	});
	stream.on('end', () => {
	resolve(Buffer.concat(chunks).toString());
	});
	stream.on('error', (err: Error) => {
	reject(err);
	});
	});
	}