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/script/script_thread.rs

@@ -945,6 +945,7 @@ impl ScriptThread {
             &js_runtime.clone(),
             PipelineId::new(),
             senders.devtools_server_sender.clone(),
+            senders.devtools_client_to_script_thread_sender.clone(),
             senders.memory_profiler_sender.clone(),
             senders.time_profiler_sender.clone(),
             script_to_constellation_chan,
@@ -2180,6 +2181,13 @@ impl ScriptThread {
             DevtoolScriptControlMsg::HighlightDomNode(id, node_id) => {
                 devtools::handle_highlight_dom_node(&documents, id, node_id)
             },
+            DevtoolScriptControlMsg::GetPossibleBreakpoints(spidermonkey_id, result_sender) => {
+                self.debugger_global.fire_get_possible_breakpoints(
+                    can_gc,
+                    spidermonkey_id,
+                    result_sender,
+                );
+            },
         }
     }