devtools: Show clients where they can set breakpoints (#37667)

devtools clients query source actors to determine where the user can set
breakpoints in a source. there are two relevant requests here:
`getBreakableLines` controls which line numbers can be clicked in the
margin, and once a line number is clicked,
`getBreakpointPositionsCompressed` controls where to show breakpoint
buttons within that line.

this patch handles those requests by querying the [SpiderMonkey Debugger
API](https://firefox-source-docs.mozilla.org/js/Debugger/) for that
information:
- devtools sends its script thread a GetPossibleBreakpoints message for
the source’s
[`spidermonkey_id`](https://firefox-source-docs.mozilla.org/js/Debugger/Debugger.Source.html#id)
- the script thread fires a `getPossibleBreakpoints` event into its
debugger global
- the debugger script looks up the
[root](https://firefox-source-docs.mozilla.org/js/Debugger/Debugger.html#onnewscript-script-global)
[Debugger.Script](https://firefox-source-docs.mozilla.org/js/Debugger/Debugger.Script.html#getpossiblebreakpoints-query)
for that source, calls
[getPossibleBreakpoints()](https://firefox-source-docs.mozilla.org/js/Debugger/Debugger.Script.html#getpossiblebreakpoints-query),
and returns the result via
DebuggerGlobalScope#getPossibleBreakpointsResult()
- that method takes the pending result sender, and sends the result back
to devtools
- devtools massages the result into the format required by the request,
and replies to the client

as a result, users of the Firefox devtools client can now set
breakpoints, though they don’t have any effect.

Testing: this patch adds new devtools tests
Fixes: part of #36027

<img width="1433" height="1328" alt="image"
src="https://github.com/user-attachments/assets/f0cd31e0-742f-44d3-8c5d-ceedd9a2706d"
/>

---------

Signed-off-by: Delan Azabani <dazabani@igalia.com>
Co-authored-by: atbrakhi <atbrakhi@igalia.com>

components/devtools/lib.rs

@@ -256,9 +256,10 @@ impl DevtoolsInstance {
                     worker_id,
                 )) => self.handle_console_message(pipeline_id, worker_id, console_message),
                 DevtoolsControlMsg::FromScript(ScriptToDevtoolsControlMsg::CreateSourceActor(
+                    script_sender,
                     pipeline_id,
                     source_info,
-                )) => self.handle_create_source_actor(pipeline_id, source_info),
+                )) => self.handle_create_source_actor(script_sender, pipeline_id, source_info),
                 DevtoolsControlMsg::FromScript(
                     ScriptToDevtoolsControlMsg::UpdateSourceContent(pipeline_id, source_content),
                 ) => self.handle_update_source_content(pipeline_id, source_content),
@@ -540,7 +541,12 @@ impl DevtoolsInstance {
         }
     }
 
-    fn handle_create_source_actor(&mut self, pipeline_id: PipelineId, source_info: SourceInfo) {
+    fn handle_create_source_actor(
+        &mut self,
+        script_sender: IpcSender<DevtoolScriptControlMsg>,
+        pipeline_id: PipelineId,
+        source_info: SourceInfo,
+    ) {
         let mut actors = self.actors.lock().unwrap();
 
         let source_content = source_info
@@ -554,6 +560,7 @@ impl DevtoolsInstance {
             source_info.content_type,
             source_info.spidermonkey_id,
             source_info.introduction_type,
+            script_sender,
         );
         let source_actor_name = source_actor.name.clone();
         let source_form = source_actor.source_form();