[Enhancement]: Surface debug.log errors

Describe the solution you'd like

RIght now the Logs panel shows a 🎉 even if there were fatals that were logged to /wordpress/wp-content/debug.log. We should surface those.

Describe alternatives you've considered

A lower-level Playground client API such as stat(path) plus readFileRange(path, offset, length) may be the cleaner long-term solution.
The PHP-side tailing sketch is a workaround that avoids broadening the client API in the short term.

Additional context

I tried an implementation in #3601 but it was getting out of hand. We don't have a playground.stat() that allows reading the file at boot to determine if something was added.

This is a potential workaround:

const debugLogMaxUnreadChars = 256 * 1024;
const debugLogReadOffsets = new WeakMap<PlaygroundClient, number>();

type DebugLogTail = {
	size: number;
	content: string;
};

async function readDebugLogTail(
	playground: PlaygroundClient
	): Promise<DebugLogTail | null> {
	const previousOffset = debugLogReadOffsets.get(playground) ?? 0;

	const response = await playground.run({
		code: `<?php
$path = '/wordpress/wp-content/debug.log';
$previousOffset = ${JSON.stringify(previousOffset)};
$maxBytes = ${JSON.stringify(debugLogMaxUnreadChars)};

if (!file_exists($path)) {
		echo json_encode(null);
		return;
}

$size = filesize($path);
$start = $previousOffset;

if ($start > $size) {
		$start = 0;
}

$start = max($start, $size - $maxBytes);

$handle = fopen($path, 'rb');
if (!$handle) {
		echo json_encode(null);
		return;
}

fseek($handle, $start);
$content = stream_get_contents($handle);
fclose($handle);

echo json_encode(array(
		'size' => $size,
			'content' => $content,
));
		`,
	});

	if (response.exitCode !== 0) {
		return null;
	}

	return response.json as DebugLogTail | null;
}

async function surfaceDebugLogEntries(playground: PlaygroundClient) {
	try {
		const tail = await readDebugLogTail(playground);
		if (!tail) {
			return;
		}

		debugLogReadOffsets.set(playground, tail.size);

		const filtered = tail.content
		.split('\n')
		.filter((line) => line.trim() && !isIgnoredLogLine(line))
		.join('\n');

		if (filtered.length > 0) {
			logger.logMessage({
				message: filtered,
				severity: LogSeverity.Log,
				raw: true,
			});
		}
	} catch {
							// Playground may not be ready yet.
	}
}

Start by tracing the Logs panel and the PlaygroundClient.run call shown in the issue; inspect how log messages are filtered and rendered. Confirm the implementation can detect new /wordpress/wp-content/debug.log content and that fatal entries appear instead of a success-only state, including when the Playground is not ready.