language: Fix language detection for injected syntax layers (#41111)

Closes #40632

**TL;DR:** The `wrap selections in tag` action was unavailable in ERB
files, even when the cursor was positioned in HTML content (outside of
Ruby code blocks). This happened because `syntax_layer_at()` incorrectly
returned the Ruby language for positions that were actually in HTML.
**NOTE:** I am not familiar with that part of Zed so it could be that
the fix here is completely incorrect.

Previously, `syntax_layer_at` incorrectly reported injected languages
(e.g., Ruby in ERB files) even when the cursor was in the base language
content (HTML). This broke actions like `wrap selections in tag` that
depend on language-specific configuration.

The issue had two parts:
1. Missing start boundary check: The filter only checked if a layer's
end was after the cursor (`end_byte() > offset`), not if it started
before, causing layers outside the cursor position to be included. See
the `BEFORE` video: when I click on the HTML part it reports `Ruby`
language instead of `HTML`.
2. Wrong boundary reference for injections: For injected layers with
`included_sub_ranges` (like Ruby code blocks in ERB), checking the root
node boundaries returned the entire file range instead of the actual
injection ranges.

This fix:
- Adds the containment check using half-open range semantics [start,
end) for root node boundaries. That ensures proper reporting of the
detected language when a cursor (`|`) is located right after the
injection:

   ```
   <body>
    <%= yield %>|
  </body>
   ```

- Checks `included_sub_ranges` for injected layers to determine if the
cursor is actually within an injection
- Falls back to root node boundaries for base layers without sub-ranges.
This is the original behavior.

Fixes ERB language support where actions should be available based on
the cursor's actual language context. I think that also applies to some
other template languages like HEEX (Phoenix) and `*.pug`. On short
videos below you can see how I navigate through the ERB template and the
terminal on the right outputs the detected language if you apply the
following patch:

```diff
diff --git i/crates/editor/src/editor.rs w/crates/editor/src/editor.rs
index 15af61f5d2..54a8e0ae37 100644
--- i/crates/editor/src/editor.rs
+++ w/crates/editor/src/editor.rs
@@ -10671,6 +10671,7 @@ impl Editor {
         for selection in self.selections.disjoint_anchors_arc().iter() {
             if snapshot
                 .language_at(selection.start)
+                .inspect(|language| println!("Detected language: {:?}", language))
                 .and_then(|lang| lang.config().wrap_characters.as_ref())
                 .is_some()
             {
```

**Before:**


https://github.com/user-attachments/assets/3f8358f4-d343-462e-b6b1-3f1f2e8c533d


**After:**



https://github.com/user-attachments/assets/c1b9f065-1b44-45a2-8a24-76b7d812130d



Here is the ERB template:

```
<!DOCTYPE html>
<html>
  <head>
    <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
    <style>
      /* Email styles need to be inline */
    </style>
  </head>
  <body>
    <%= yield %>
  </body>
</html>
```

Release Notes:

- N/A

crates/language/src/buffer.rs

@@ -3366,7 +3366,19 @@ impl BufferSnapshot {
     pub fn syntax_layer_at<D: ToOffset>(&self, position: D) -> Option<SyntaxLayer<'_>> {
         let offset = position.to_offset(self);
         self.syntax_layers_for_range(offset..offset, false)
-            .filter(|l| l.node().end_byte() > offset)
+            .filter(|l| {
+                if let Some(ranges) = l.included_sub_ranges {
+                    ranges.iter().any(|range| {
+                        let start = range.start.to_offset(self);
+                        start <= offset && {
+                            let end = range.end.to_offset(self);
+                            offset < end
+                        }
+                    })
+                } else {
+                    l.node().start_byte() <= offset && l.node().end_byte() > offset
+                }
+            })
             .last()
     }