Remove code snippets instead of redacting them in trimming

[calyptobai]

Remove code chunks instead of redacting them. This should hopefully reduce the frequency at which we see [REDACTED] in model output.

Closes BLO-1842

[ggordonhall]

In testing this works well, but this is largely because the model rarely generates either QuotedCode or GeneratedCode: it prefers markdown code blocks so it's tricky to evaluate the effects of this change.

[ggordonhall]

Do we need to do this work at all if we're returning an empty string?

[calyptobai]

We do need to do this for partially generated messages. E.g. if a message generates with <GeneratedCode><Code>foo, we need to call fixup_xml_code to complete the block so that it can still be parsed on the line right below. If parsing fails here (e.g. there is a different kind of XML block, perhaps HTML in the markdown), we know the block is not a code chunk and should be kept.