When one asks if something is free software (a.k.a. FOSS) the concern isn't so much trust but rather can one view, modify, and share the program. Sandboxes solve a different problem.

In the case of a javascript bundle, in order for a user to exercise the Four Freedoms they must at minimum be provided with corresponding source code for each component in the bundle, and preferably some way in the browser for the user to inspect and modify it. In other words, it must be treated like any other compiled binary program. A lock file with specific versions probably isn't necessary (and server configuration and source code definitely isn't).

You are right in that this would require cooperation from the service provider to provide this metadata, and most definitely would not do this. Therefore, such an extension as OP suggests would have the effect of blocking the vast majority of javascript on the web today. LibreJS tries to some extent but I don't know how well it can handle bundled javascript files.