fix(openapi): omit header and cookie params the model leaves null

[Kumaresan-AI-Engineer]

A model routinely emits null for an optional parameter it decided not to use. _prepare_request_params already drops such a value for a query parameter, but passed it through for header and cookie parameters:

• httpx refuses to encode a None header value, so the whole tool call fails with "Header value must be str or bytes, not <class 'NoneType'>" before the request is ever sent, and the model gets no usable error to recover from.
• httpx accepts a None cookie and serializes it as a bare, valueless Cookie: <name> pair, so the request goes out malformed and silently.

Skip both when the value is None, matching how an unset query parameter is already handled in the same loop. An explicitly empty string is still sent, so a caller can keep passing a deliberate empty value.

[google-cla]

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