Improve processing of `content` argument that's passed in to `generate_content` method

[ericapisani]

While working through PY-2287, it looks like there may be a few gaps here.

Audit the following to ensure that we have complete coverage of these data types:

• FileDict
• PartDict forms that involve file_data, function_call, executable_code, and code_execution_result

[linear-code]

PY-2289