Avoid spurious breakpoint-setting failure in DAP

A user pointed out that if a DAP setBreakpoints request has a 'source'
field in a SourceBreakpoint object, then the gdb DAP implementation
will throw an exception.

While SourceBreakpoint does not allow 'source' in the spec, it seems
better to me to accept it.  I don't think we should fully go down the
"Postel's Law" path -- after all, we have the type-checker -- but at
the same time, if we do send errors, they should be intentional and
not artifacts of the implementation.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30820

gdb/python/lib/gdb/dap/breakpoint.py

@@ -273,7 +273,13 @@ def set_breakpoint(*, source, breakpoints: Sequence = (), **args):
     if "path" not in source:
         result = []
     else:
-        specs = [_rewrite_src_breakpoint(source=source, **bp) for bp in breakpoints]
+        # Setting 'source' in BP avoids any Python error if BP already
+        # has a 'source' parameter.  Setting this isn't in the spec,
+        # but it is better to be safe.  See PR dap/30820.
+        specs = []
+        for bp in breakpoints:
+            bp["source"] = source
+            specs.append(_rewrite_src_breakpoint(**bp))
         # Be sure to include the path in the key, so that we only
         # clear out breakpoints coming from this same source.
         key = "source:" + source["path"]