language: Make Buffer::new take an explicit ID (#2900)

See Linear description for the full explanation of the issue. This PR is
mostly a mechanical change, except for the one case where we do pass in
an explicit `next_id` instead of `model_id` in project.rs.

Release Notes:
- Fixed a bug where some results were not reported in project search in
presence of unnamed buffers.

crates/zed/src/languages/c.rs

@@ -289,7 +289,7 @@ mod tests {
         let language = crate::languages::language("c", tree_sitter_c::language(), None).await;
 
         cx.add_model(|cx| {
-            let mut buffer = Buffer::new(0, "", cx).with_language(language, cx);
+            let mut buffer = Buffer::new(0, cx.model_id() as u64, "").with_language(language, cx);
 
             // empty function
             buffer.edit([(0..0, "int main() {}")], None, cx);

crates/zed/src/languages/python.rs

@@ -210,7 +210,7 @@ mod tests {
         });
 
         cx.add_model(|cx| {
-            let mut buffer = Buffer::new(0, "", cx).with_language(language, cx);
+            let mut buffer = Buffer::new(0, cx.model_id() as u64, "").with_language(language, cx);
             let append = |buffer: &mut Buffer, text: &str, cx: &mut ModelContext<Buffer>| {
                 let ix = buffer.len();
                 buffer.edit([(ix..ix, text)], Some(AutoindentMode::EachLine), cx);

crates/zed/src/languages/rust.rs

@@ -474,7 +474,7 @@ mod tests {
         let language = crate::languages::language("rust", tree_sitter_rust::language(), None).await;
 
         cx.add_model(|cx| {
-            let mut buffer = Buffer::new(0, "", cx).with_language(language, cx);
+            let mut buffer = Buffer::new(0, cx.model_id() as u64, "").with_language(language, cx);
 
             // indent between braces
             buffer.set_text("fn a() {}", cx);

crates/zed/src/languages/typescript.rs

@@ -356,8 +356,9 @@ mod tests {
         "#
         .unindent();
 
-        let buffer =
-            cx.add_model(|cx| language::Buffer::new(0, text, cx).with_language(language, cx));
+        let buffer = cx.add_model(|cx| {
+            language::Buffer::new(0, cx.model_id() as u64, text).with_language(language, cx)
+        });
         let outline = buffer.read_with(cx, |buffer, _| buffer.snapshot().outline(None).unwrap());
         assert_eq!(
             outline