Combine excerpt footer and header into a single block (#19441)

This simplifies rendering of excerpt headers and footers, and removes
the need to store a `BlockDisposition` on these boundary blocks. It's a
step toward implementing "replace blocks", which we want to use in the
assistant panel.

We've also cleaned up the way heights are specified for headers and
footers and fixed some visual asymmetries between the "expand upward"
and "expand downward" buttons.

Release Notes:

- N/A

---------

Co-authored-by: Richard <[email protected]>

crates/diagnostics/src/diagnostics_tests.rs

@@ -962,7 +962,6 @@ fn random_diagnostic(
 
 const FILE_HEADER: &str = "file header";
 const EXCERPT_HEADER: &str = "excerpt header";
-const EXCERPT_FOOTER: &str = "excerpt footer";
 
 fn editor_blocks(
     editor: &View<Editor>,
@@ -998,7 +997,7 @@ fn editor_blocks(
                                     .ok()?
                             }
 
-                            Block::ExcerptHeader {
+                            Block::ExcerptBoundary {
                                 starts_new_buffer, ..
                             } => {
                                 if *starts_new_buffer {
@@ -1007,7 +1006,6 @@ fn editor_blocks(
                                     EXCERPT_HEADER.into()
                                 }
                             }
-                            Block::ExcerptFooter { .. } => EXCERPT_FOOTER.into(),
                         };
 
                         Some((row, name))