feat(otel): Instrument dynamic workflows

[RKest]

Problem:
Children executed via workflow.RunNode inside a DynamicNode were not instrumented, so RunNode-driven delegation was invisible in the trace tree.

Solution:
dynamicSubScheduler.runNode now opens an invoke_node span per delegated child, nested under the dynamic node's span, and records genuine runtime failures while skipping expected control flow (context.Canceled, ErrNodeInterrupted) to match the top scheduler. The telemetry schema test case was replaced (WorkflowChainCase → WorkflowDynamicCase) to assert the nested span shape.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

ok  google.golang.org/adk/workflow                            0.111s
ok  google.golang.org/adk/internal/telemetry                  (cached)
ok  google.golang.org/adk/internal/telemetry/functionaltest   0.030s

Manual End-to-End (E2E) Tests:

go run ./examples/workflow/dynamic/hitl/main.go console -otel_to_cloud

Dynamic node is instrumented:

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

Additional context

N/A

[wolo-lab]

we have two workflow types: static and dynamic. I think we should cover both of them, so just add a new test instead of replacing it.

[wolo-lab]

it would be nice to have some more tests with error handling

1. e.g. upper_node returning error.
2. check that ErrNodeInterrupted is properly handled

[wolo-lab]

just to double check that it is desired behavior: cache lookup won't emit span. If so we could add a small comment so in the future we would know that it is ok.

[wolo-lab]

super nit: result name is used but in the code we have return **out**, nil. It would be a bit more consistent if we use out or result in both places: in the function signature and in the body