Large tool results not checked for content length if using non-string content blocks

[bracesproul]

the logic which checks the size of the tool result only considers content blocks which are strings, not multi-type content blocks

this means if you have a complex content block with a text field that contains 1 billion characters, it'll get sent right back to the agent

i think the fix here is to join all content blocks with type text, and check the lengths of those

libs/deepagents/src/middleware/fs.ts L857