Relax client tool call output request thread items lookback from 1 to default page size, bump patch version to 1.6.4

[jiwon-oai]

When ThreadsAddClientToolOutputReq() is invoked, we raise a value error and abort the stream server-side unless the very last thread item in the current thread is a pending client tool call item.

This is fragile for cases where a previous turn might add hidden context items or items added with the same created at timestamp can have swapped orders etc.

Relaxing this requirement so that:

• We look back a page size (20) instead of just 1 item
• We interpret the request as a "resume" signal and continue inference even if a pending tool call wasn't found.

This lets us handle a case that would've always been an error to be resilient and continue the response instead of flopping.

Tested against chatkit studio by manually appending a hidden context item after streaming a client tool call item:

Status quo	After relaxing requirements
Stream abruptly stops after error	Stream continues