fix(config): align additional_properties typing

[hariharan077]

Description

Fixes #5268.

Declare generated config model additional_properties attributes as instance dataclass fields instead of ClassVars, matching the runtime behavior of @_additional_properties and the _ComponentConfig protocol.

This also skips non-init dataclass fields while resolving configured resource detectors, so the generated additional_properties field is not treated as a detector name.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• Reproduced the strict pyright failure locally with a temporary SpanExporter/_resolve_component repro before the fix
• Confirmed the same strict pyright repro passes after the fix with 0 errors, 0 warnings, 0 informations
• .tox/py310-test-opentelemetry-sdk/bin/python -m pytest opentelemetry-sdk/tests/_configuration/test_common.py opentelemetry-sdk/tests/_configuration/test_resource.py -q
• uv run tox -r -e typecheck
• uv run tox -r -e lint-opentelemetry-sdk
• uv run ruff check opentelemetry-sdk/src/opentelemetry/sdk/_configuration/models.py opentelemetry-sdk/src/opentelemetry/sdk/_configuration/_common.py opentelemetry-sdk/src/opentelemetry/sdk/_configuration/_resource.py opentelemetry-sdk/tests/_configuration/test_common.py
• uv run ruff format --check opentelemetry-sdk/src/opentelemetry/sdk/_configuration/models.py opentelemetry-sdk/src/opentelemetry/sdk/_configuration/_common.py opentelemetry-sdk/src/opentelemetry/sdk/_configuration/_resource.py opentelemetry-sdk/tests/_configuration/test_common.py
• git diff --check

Does This PR Require a Contrib Repo Change?

• No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

Changelog and codegen README documentation are included for the additional_properties field design.

[MikeGoldsmith]

Thanks @hariharan077 - sorry for the slow review.

There's a couple of things that need fixing:

• We're still importing typing.ClassVar - I think we should be using dataclasses.field instead
• The README.md needs updating with the new design, the field is now an init=False dataclass field with default_factory=dict and the @_additional_properties decorator assigns the extras in the constructor
• You need to add a changelog fragment, something like .changelog/5286.fixed

[hariharan077]

Addressed the review comments in ebb349e: updated the codegen README for the init=False/default_factory additional_properties field, added .changelog/5286.fixed, and refreshed the PR checklist. Local focused tests, SDK lint, ruff, and towncrier draft all pass.