Python: apply function_invocation_configuration in BaseChatClient.as_agent

[Bortlesboat]

Motivation and Context

Fixes #5180.

BaseChatClient.as_agent() currently accepts function_invocation_configuration but forwards it into Agent(**agent_kwargs), where Agent.__init__() does not accept that keyword. In practice this raises a TypeError instead of applying the configuration to function-invoking clients.

Description

This change keeps the override on the client side, which is where FunctionInvocationLayer consumes it:

• stop passing function_invocation_configuration into Agent()
• when the client supports FunctionInvocationLayer, normalize and apply the override directly to self.function_invocation_configuration
• add a regression test covering as_agent(function_invocation_configuration=...) so the issue stays fixed

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[eavanvalkenburg]

just added a note on the original issue, that this is not the way to fix that issue, if you are up for it, you can change this PR, open a new PR or we can pick it up with the team