Do not serialize buffers containing bundled files (#38102)

Fix bundled file persistence by introducing SerializationMode enum

Closes: #38094

What's the issue?

Opening bundled files like Default Key Bindings
(zed://settings/keymap-default.json) was causing SQLite foreign key
constraint errors. The editor was trying to save state for these
read-only assets just like regular files, but since bundled files don't
have database entries, the foreign key constraint would fail.

The fix

Replaced the boolean serialize_dirty_buffers flag with a type-safe
SerializationMode enum:

```rust
pub enum SerializationMode {
    Enabled,   // Regular files persist across sessions
    Disabled,  // Bundled files don't persist
}
```

This prevents serialization at the source: workspace_id() returns None
for disabled editors, serialize() bails early, and should_serialize()
returns false. When opening bundled files, we set the mode to Disabled
from the start, so they're treated as transient views that never
interact with the persistence layer.

Changes

- editor.rs: Added SerializationMode enum and updated serialization
methods to respect it
- items.rs: Guarded should_serialize() to prevent disabled editors from
being serialized
- zed.rs: Set SerializationMode::Disabled in open_bundled_file()

Result

Bundled files open cleanly without SQLite errors and don't persist
across workspace reloads (expected behavior). Regular file persistence
remains unaffected.

Release Notes: Fixed SQLite foreign key constraint errors when opening
bundled files like Default Key Bindings.

---------

Co-authored-by: MrSubidubi <finn@zed.dev>

crates/zed/src/zed.rs

@@ -1922,6 +1922,7 @@ fn open_bundled_file(
                             let mut editor =
                                 Editor::for_multibuffer(buffer, Some(project.clone()), window, cx);
                             editor.set_read_only(true);
+                            editor.set_should_serialize(false, cx);
                             editor.set_breadcrumb_header(title.into());
                             editor
                         })),