Improve syntax error reporting by adding file context

When syntax errors occur during file loading, the error messages now
include the source file name through call_with_error_context/2 as
recommended in issue mthom#302.

Before:
  error(syntax_error(incomplete_reduction),read_term/3:8).

After:
  error(syntax_error(incomplete_reduction),[file-'test_file.pl'|read_term/3:8]).

This approach uses the existing call_with_error_context/2 mechanism
instead of modifying the core error struct, following the established
pattern for enhancing error context in Scryer Prolog.

Changes:
- Modified stream_next_term/4 in src/loader.pl to wrap read_term/3
  calls with call_with_error_context/2, providing file context when
  available

Resolves: mthom#302
Related: mthom#3133

Author: jjtolton

src/loader.pl

@@ -201,8 +201,14 @@
 
 stream_next_term(Stream, Term, LinesRead, Singletons) :-
     (  '$devour_whitespace'(Stream) ->
-       stream_property(Stream, position(position_and_lines_read(_, LinesRead))),
-       read_term(Stream, Term, [singletons(Singletons)])
+       stream_property(Stream, position(position_and_lines_read(Pos0, LinesRead))),
+       (  prolog_load_context(file, File) ->
+          call_with_error_context(
+              read_term(Stream, Term, [singletons(Singletons)]),
+              file-File
+          )
+       ;  read_term(Stream, Term, [singletons(Singletons)])
+       )
     ;  Term = end_of_file
     ).