fix(types): parameterize Span's context manager and annotate otel span processor

[Adam Hitchcock (NorthIsUp)]

Consumers that run pyright in strict mode see reportUnknown* warnings at every start_span call and every with ... as span block, because:

• Span subclasses a bare contextlib.AbstractContextManager, so __enter__ returns Unknown and span is untyped inside the with block. Now AbstractContextManager["Span"]; both implementations already return self.
• start_span's parent: str | dict | None uses a bare dict. Now dict[str, str], which is what extract_trace_context builds (its return type is narrowed to match).
• add_braintrust_span_processor leaves tracer_provider and custom_filter unannotated. Now TracerProvider and Callable[[ReadableSpan], bool | None], imported under TYPE_CHECKING so the optional otel dependency stays optional.
• _MemoryBackgroundLogger.pop gets its list[dict[str, Any]] return type (what merge_row_batch returns).

No runtime change. Adds type_tests/test_span_types.py; its assert_type(span, Span) fails on main ("expected Span but received Unknown") and passes here under both pyright and mypy. Repo-root pyright on the two touched files goes from 239 to 228 errors and 656 to 625 warnings.

Checks run: nox test_types steps (pyright + mypy + pytest on type_tests), pytest src/braintrust/test_logger.py, ruff 0.15.21 check and format.

[Abhijeet Prasad (AbhiPrasad)]

released with https://github.com/braintrustdata/braintrust-sdk-python/releases/tag/py-sdk-v0.44.1. - thanks for the PR Adam Hitchcock (@NorthIsUp)