fix(otel): end recording spans on non-terminal

[ayushiahjolia]

Issue #, if available:
#642

Description of changes:

• Both OTel plugins abandoned recording spans without ending them when a durable execution returned a non-terminal status, violating the OTel requirement that every created span be ended.
• The execution plugin dropped open operation spans on PENDING/RETRY, and both plugins dropped the execution-scoped Workflow span on non-terminal invocations.
• Replace the recording Workflow span with a non-recording deterministic placeholder held during the invocation.
• The placeholder still carries the deterministic Workflow SpanContext - now on the shared execution trace, parented to the execution ancestor - so it parents and links operation spans, keeps the Workflow current for auto-instrumented spans, and serves log correlation, without leaving a recording span to abandon.
• The recording Workflow span is created and ended exactly once, only on a terminal status, anchored at the execution start time.
• The execution plugin now also ends any operation spans still open at invocation end, matching the invocation plugin. Cross-invocation correlation is unchanged: an operation that resumes later still gets its authoritative span from on_operation_end.

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