core[minor]: StringOutputParser and text BaseMessage contents

[afirstenberg]

Added a number of methods to BaseLLMOutputParser and call from invoke() to allow subclasses to override the string representation of a BaseMessage. Take advantage of this as part of StringOutputParser to make sure any text types are represented as human-readable strings.

Fixes #4695

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[jacoblee93]

Thank you for this! The silent failure/truncation on complex message image outputs makes me a bit nervous though. Happy to make the change myself.

[afirstenberg]

I have no strong feelings, but here's my justification for choosing an empty string:

• Converting something to a displayable string should never error out. There should always be SOMETHING to display to the user
• One could argue if that should be "" or "[IMG]" or something else.
• It is easy to override (it is in it's own function)
• I don't think any model is sending this back right now, but when a model does, it shouldn't break existing code.

That said, as long as the _imageUrlContentToString() function is the one throwing the exception, so it can be overridden - I have no objections. (May also need to add something to the default switch statement as well to handle any future undefined types. I'm better with that being an exception since there are no other types defined today.)

[afirstenberg]

If we're throwing errors - should probably also throw an error here. (For any future content types that may be created. Marking an unknown content type.)

[jacoblee93]

Yes good call

[jacoblee93]

Thanks very much!