windows: Fix inconsistent separators in buffer headers and breadcrumbs (#38898)

Make `resolve_full_path` use the appropriate separators, and return a
`String`.

As part of fixing the fallout from that type change, this also fixes a
bunch of places in the agent code that were using `std::path::Path`
operations on paths that could be non-local, by changing them to operate
instead on strings and use the project's `PathStyle`.

This clears the way a bit for making `full_path` also return a string
instead of a `PathBuf`, but I've left that for a follow-up.

Release Notes:

- N/A

crates/agent2/src/tools/read_file_tool.rs

@@ -225,9 +225,12 @@ impl AgentTool for ReadFileTool {
                 Ok(result.into())
             } else {
                 // No line ranges specified, so check file size to see if it's too big.
-                let buffer_content =
-                    outline::get_buffer_content_or_outline(buffer.clone(), Some(&abs_path), cx)
-                        .await?;
+                let buffer_content = outline::get_buffer_content_or_outline(
+                    buffer.clone(),
+                    Some(&abs_path.to_string_lossy()),
+                    cx,
+                )
+                .await?;
 
                 action_log.update(cx, |log, cx| {
                     log.buffer_read(buffer.clone(), cx);