feat(otel): parent durable spans to shared trace

[ayushiahjolia]

Issue #, if available: #674

Description of changes:

• Third PR in the stack for reparenting durable OTel spans onto one shared execution trace.
• Wires both plugins (InvocationOtelPlugin and ExecutionOtelPlugin) onto the shared execution trace and sampling. Workflow and Invocation spans now share one trace, anchored to a propagated backend parent when present or a deterministic synthetic execution root otherwise; an ambient Lambda span is used as parent only when it already belongs to the execution trace.
• Sampling is resolved once per invocation and applied to every durable span. Also completes the context_extractors rewrite deferred from the first PR: extractors now return a structured ExtractedContext instead of an opaque OTel Context.
• Next in stack: docs (README) update for the shared-trace model.

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[ayushiahjolia]

/ai review