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* Why?

We would like the ability to configure different parser types. Prior to
this commit, only the JSON parser could be configured.

* What?

This commit refactors the tool parser config in the following ways:
- the `format` and `json` fields of `ToolParserConfig` are merged into
  a single `config` field that is a "discriminated union" type. Each
  parser type can declare its own configuration options.
- a `XmlParserConfig` is defined with a default factory method that
  corresponds to the Qwen3 coder configuration.
- affected calls and tests are adjusted.