[roll] Roll fuchsia [check-licenses] Fix concurrency and panic edge cases in file package

This change addresses several stability issues within the core `file`
package used for discovering and analyzing license files. And it adds a
bunch of much needed tests.

**Concurrency:** Added a `sync.Mutex` to the `File` struct to protect
the unexported `data` and `text` fields. `LoadContent()` and
`UnloadContent()` now correctly acquire the lock, eliminating duplicate
disk reads and data races when multiple goroutines process the same
file.

**Nil Pointer Dereference:** Added a safeguard in `LicenseType()` to
check if `d.searchResults` is nil before iterating over matches. This
prevents runtime panics if the function is called before `Search()` has
populated the results.

**Nil Map Assignment:** Updated `FileConfig.Merge()` to explicitly
initialize the `Extensions` map if it was unmarshaled as nil, preventing
a panic during configuration merging.

Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1566539
Original-Revision: cc7898e13450091b9e4100fb0643d834ed0fc4ab
GitOrigin-RevId: dac568eee7658bfad71c721c0f07b333310d8483
Change-Id: I7cd99b38c619ca667f53a8b700becddc6d7d35d6