Revisit FunctionInfoResult type in profile-query library

`FunctionInfoResult` has a `library` property, but after #6258, a single function can have frames from multiple libraries. At the moment we return an arbitrary one. We should revisit this.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-1406)

Start by locating FunctionInfoResult in the profile-query library and tracing its callers to understand how the library property is consumed. Review the behavior introduced by #6258; done means the result type and its consumers no longer rely on an arbitrary library when a function has frames from multiple libraries, with tests covering that case.

Written by the indexing model from the issue text.