Rollup merge of rust-lang#140076 - aDotInTheVoid:jsondocline, r=GuillaumeGomez

jsondocck: Require command is at start of line

In one place we use `///``@``` instead of `//``@`.`` The test-runner allowed it, but it probably shouldn't. Ran into by ``@lolbinarycat`` in rust-lang#132748 (comment):

```
error: unknown disambiguator `?(`
##[error] --> /checkout/tests/rustdoc-json/fns/return_type_alias.rs:3:25
  |
3 | ///@ set foo = "$.index[?(``@.name=='Foo')].id"``
  |                         ^^
  |
```

Maybe it's also worth erroring on this like we added in rust-lang#137103

r? ``@GuillaumeGomez``

Author: ChrisDenton

src/tools/jsondocck/src/main.rs

@@ -154,6 +154,7 @@ impl CommandKind {
 static LINE_PATTERN: LazyLock<Regex> = LazyLock::new(|| {
     RegexBuilder::new(
         r#"
+        ^\s*
         //@\s+
         (?P<negated>!?)
         (?P<cmd>[A-Za-z0-9]+(?:-[A-Za-z0-9]+)*)

tests/rustdoc-json/fns/return_type_alias.rs

@@ -1,6 +1,6 @@
 // Regression test for <https://github.com/rust-lang/rust/issues/104851>
 
-///@ set foo = "$.index[?(@.name=='Foo')].id"
+//@ set foo = "$.index[?(@.name=='Foo')].id"
 pub type Foo = i32;
 
 //@ is "$.index[?(@.name=='demo')].inner.function.sig.output.resolved_path.id" $foo