Decide and document how file name extensions are handled in MemoryStorage KeyValueStore

[janbuchar]

• follow up to https://apify.slack.com/archives/C02JQSN79V4/p1759231330021089
• Python FileSystemKeyValueStoreClient currently does not automatically append extensions to the file names (keys)
• In JS, it does -

  crawlee/packages/memory-storage/src/resource-clients/key-value-store.ts

  Lines 283 to 302 in 8c8e1f4

  	const isValueStreamOrBuffer = valueIsStream || isBuffer(value);
  	// To allow saving Objects to JSON without providing content type
  	if (!contentType) {
  	if (isValueStreamOrBuffer) contentType = 'application/octet-stream';
  	else if (typeof value === 'string') contentType = 'text/plain; charset=utf-8';
  	else contentType = 'application/json; charset=utf-8';
  	}
  	const extension = mime.extension(contentType) || DEFAULT_LOCAL_FILE_EXTENSION;
  	const isContentTypeJson = extension === 'json';
  	if (isContentTypeJson && !isValueStreamOrBuffer && typeof value !== 'string') {
  	try {
  	value = JSON.stringify(value, null, 2);
  	} catch (err: any) {
  	const msg = `The record value cannot be stringified to JSON. Please provide other content type.\nCause: ${err.message}`;
  	throw new Error(msg);
  	}
  	}

• This behavior needs to be unified and documented in both flavors
• There was a concern that the Apify Key-value store does not accept extensions in the keys, but this has been proven false
  • in fact, many well-known Actors specify the extensions explicitly
• Adding extensions automatically on the other hand can be considered user friendly for those who want to open the "values" using common editors/viewers