Remove leaf_customer_span_id baggage re-parenting hack from tracing

[needuv]

Use standard OTel traceparent propagation instead of overriding the parent span ID via the baggage header's leaf_customer_span_id key.

• Remove _parse_baggage_key() and _override_parent_span_id() helpers
• Remove baggage_header parameter from span/start_span/_extract_context
• Simplify _prepare_request_span_args to return 3-tuple (no baggage)
• Remove associated tests (TestParseBaggageKey, test_baggage_leaf_customer_span_id)
• Preserve set_baggage/detach_baggage OTel context API (unrelated)

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