Prior to this commit, if a PathContainer was created using
Options.MESSAGE_ROUTE, DefaultPathSegment#parameters() returned a
mutable map which would allow the user to modify the contents of the
static, shared EMPTY_PARAMS map in DefaultPathContainer.
This commit prevents corruption of the shared EMPTY_PARAMS map by
ensuring that parameters stored in DefaultPathSegment are always
immutable.
Closes gh-27064
3676084472