Support upstream traceparent headers

[adagradschool]

Product context

This lets external callers stitch Claude Code runs into an existing trace by passing a W3C traceparent. That makes it possible to group multiple Claude Code sessions under one orchestrated trace instead of always starting a new root trace in the plugin.

Introduced changes

• Added PROMPTLAYER_TRACEPARENT support in the trace plugin
• Reused the upstream trace ID when provided
• Emitted the Claude Code session span as a child of the upstream parent span ID
• Preserved existing behavior when no upstream context is passed
• Added a spec reference doc for the W3C Trace Context sections this change follows

Edge cases covered

• Missing traceparent falls back to a generated trace
• Invalid traceparent falls back to a generated trace
• Valid 00 headers are accepted
• Non-00 headers are accepted for forward compatibility
• Non-00 headers with an extra suffix do not break parsing

Verification

• make test passed