assistant edit tool: Reliability improvements (#27431)

- Add instructions in description to read before editing
- Add instructions in edit prefix to explicitly ask for reads
- Fix `connection error: delay between messages too long` by processing
chunks off a channel

Release Notes:

- N/A

crates/assistant_tools/src/edit_files_tool/description.md

@@ -5,3 +5,5 @@ When using this tool, you should suggest one coherent edit that can be made to t
 When the set of edits you want to make is large or complex, feel free to invoke this tool multiple times, each time focusing on a specific change you wanna make.
 
 You should use this tool when you want to edit a subset of a file's contents, but not the entire file. You should not use this tool when you want to replace the entire contents of a file with completely different contents, and you absolutely must never use this tool to create new files from scratch. If you ever consider using this tool to create a new file from scratch, for any reason, instead you must reconsider and choose a different approach.
+
+DO NOT call this tool until the code to be edited appears in the conversation! You must use the `read-files` tool or ask the user to add it to context first.

crates/assistant_tools/src/edit_files_tool/edit_prompt.md

@@ -120,7 +120,7 @@ Break large *SEARCH/REPLACE* blocks into a series of smaller blocks that each ch
 Include just the changing lines, and a few surrounding lines if needed for uniqueness.
 Do not include long runs of unchanging lines in *SEARCH/REPLACE* blocks.
 
-Only create *SEARCH/REPLACE* blocks for files that the user has added to the chat!
+Only create *SEARCH/REPLACE* blocks for files that have been read! Even though the conversation includes `read-file` tool results, you *CANNOT* issue your own reads. If the conversation doesn't include the code you need to edit, ask for it to be read explicitly.
 
 To move code within a file, use 2 *SEARCH/REPLACE* blocks: 1 to delete it from its current location, 1 to insert it in the new location.