feat(config): recursively convert parsed dicts to typed dataclasses in loader

[MikeGoldsmith]

Description

Closes the gap between load_config_file() and the factory functions: YAML/JSON config → SDK objects now works end-to-end through the typed model tree.

Previously, the loader's _dict_to_model did OpenTelemetryConfiguration(**data) which only constructed the top-level dataclass — nested fields stayed as raw dicts. This meant factory functions like create_tracer_provider(config: TracerProviderConfig) would break trying to access config.sampler as an attribute when it was actually a dict.

Approach

Added _dict_to_dataclass in a new _conversion.py module. It walks each field's type annotation via typing.get_type_hints and recursively converts:

• Nested dicts → typed dataclass instances (e.g. dict → TracerProvider → SpanProcessor → BatchSpanProcessor → ...)
• Lists of dicts → lists of typed dataclasses (list[SpanProcessor])
• String/value → Enum members (e.g. log_level: info → SeverityNumber.info)
• Unknown keys → captured by the @_additional_properties decorator (so user-defined plugin names still flow through)

Optional[X] / X | None is unwrapped before checking the inner type. ClassVar fields are skipped (the additional_properties annotation on decorated classes is correctly ignored).

Verified end-to-end

yaml_data = '''
file_format: '1.0-rc.1'
tracer_provider:
  processors:
    - simple:
        exporter:
          console: {}
  sampler:
    parent_based:
      root:
        trace_id_ratio_based: {ratio: 0.5}
  limits:
    attribute_count_limit: 10
'''
config = _dict_to_model(yaml.safe_load(yaml_data))
provider = create_tracer_provider(config.tracer_provider)
# → TracerProvider with ParentBased sampler, 10 attribute limit, etc.

User-defined plugins continue to work — unknown sampler/propagator/exporter names land in additional_properties and are loaded via entry points.

Tests

11 new tests in test_conversion.py covering: flat dicts, nested dataclasses, lists, optionals, missing fields, unknown keys (additional_properties), enum coercion, primitive pass-through.

Closes #5127