Fix parsing anthropic overloaded error

[thdxr]

addresses #2453

it seems that for some reason the anthropic api is returning "details: null" in the error response for overloaded

{"type":"error","error":{"details":null,"type":"overloaded_error","message":"Overloaded"}}

this allows for parsing that properly and fixes retries being broken

[lgrammel]

this fix does not help with parsing. wrote a test that shows that the provided snippet parses fine #7323 . can you provide a reproduction?

[thdxr]

ah sorry about that thought i saw a .strict() call somewhere

let me try this again - hard to figure out because we can't force anthropic to produce an overloaded call