Fix generated config additional_properties typing for pyright strict

[saisharan0103]

Generated configuration dataclasses currently annotate additional_properties as ClassVar[dict[str, Any]], while _additional_properties creates and assigns self.additional_properties per instance after dataclass initialization. _ComponentConfig models that runtime contract as an instance attribute, so pyright strict mode rejects the generated config classes as structurally incompatible with the protocol even though standard mode permits the mismatch.

This changes the datamodel-codegen template to emit additional_properties as an instance dataclass field using field(default_factory=dict) with the more precise dict[str, dict[str, Any] | None] value type. The generated model imports are updated to use field instead of ClassVar, and the checked-in models.py is updated to match the template.

The stale _ComponentConfig docstring note about the intentional mismatch is removed. Local test fixture dataclasses now use the same instance-field typing, with a regression assertion that generated models expose additional_properties as a dataclass instance field with independent default dictionaries.

Fixes #5268

ruff check opentelemetry-configuration/codegen/dataclass.jinja2 opentelemetry-configuration/src/opentelemetry/configuration/_common.py opentelemetry-configuration/src/opentelemetry/configuration/models.py opentelemetry-configuration/tests/test_common.py pyproject.toml reports no new findings on the changed files.
Ran pytest -x locally with no new failures.

[linux-foundation-easycla]

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