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In <https://github.com/ollama/ollama/issues/12357> we that the model
will output tool calls such as

```
<function=shell>
<parameter=command>
pwd && ls -la
</parameter>
</function>
```

We parse this using the approach of transforming into valid xml and then
using an xml parser. While we do transform the function and parameter
names, we weren't escaping the parameter values (which in this example
are invalid since `pwd && ls -la` contains unescaped ampersands).

This has been fixed by first transforming the tags in the same way, and
then walking the transformed string and escaping the text in between the
tags. This also fixes a case where `<` in the middle of a parameter
value would cause an xml parse failure.

Fixes: #12357