fix(http-client-python): do not precompute Model class as XML deserializer

[l0lawrence]

Regression from #10698.

When the customized class is instantiated first, Model.__new__ walks the MRO and sets rf._type from the last-wins annotation (the custom class). The _RestField is shared with the generated parent, so the parent's rf._type ends up pointing at partial(_deserialize_model, CustomRetentionPolicy). _build_xml_field_plan extracts that and stores CustomRetentionPolicy as the precomputed deser for the generated parent's field plan. _init_from_xml then calls CustomRetentionPolicy(<ET.Element>) directly, the element is bound positionally to enabled (truthy), days is None, and you get the ValueError.

Before #10698 the same path went through _deserialize -> _deserialize_default, whose except Exception: pass swallowed the ValueError and returned the raw ET.Element for the consumer's _from_generated shim to handle.

Fix: stop precomputing Model and Optional[Model] fields in _build_xml_field_plan. Scalar rest_field(deserializer=...) callables are still precomputed. Model fields fall back to _deserialize(rf._type, item) like before. Also removed the now-unused _extract_xml_model_type helper.

Added a regression test that reproduces the pattern. It fails on current code and passes with the fix. Two existing tests that were also broken by the same precomputation (test_polymorphic_deserialization, test_enumeration_results_blobs_unwrapped) start passing as a side effect. All 375 unit tests pass after regen.

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commit: fb1839b

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All changed packages have been documented.

• ✅ @typespec/http-client-python

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@typespec/http-client-python - fix ✏️

Fix XML deserialization regression where a downstream customization that subclassed a generated Model and re-annotated a nested Model field with a strict subclass would cause the generated parent class's _init_from_xml to call CustomModel(<ET.Element>) directly and raise. The precomputed XML field plan no longer hard-codes the Model class as the deserializer, deferring to the generic _deserialize path that tolerates such customizations.

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[iscai-msft]

what's the perf regression by no longer precomputing the model class?

[microsoft-github-policy-service]

Hi @@l0lawrence. Your PR has had no update for 14 days and it is marked as a stale PR. If it is not updated within 14 additional days, the PR will automatically be closed. If you want to refresh the PR, please remove the stale label.