[Lancetnik]

Why are these changes needed?

This PR activates observable subagent trees, addressed delivery, cancellation cleanup, and source-event accounting. It removes legacy rollup generation and migrates UI, observers, telemetry, eval, network consumers, and their tests together. Delegation, accounting, HITL, cancellation, and consumer regressions accompany the implementation.

Addressed delivery has one public operation: event.origin.send(...). Only the StreamHandle and DeliveryError types are added to the existing subagent package exports. send_down and registry lookup/broadcast by task ID, agent name, or subtree are removed. run_task, TaskResult, and DelegationRegistry remain internal and are not re-exported. Registry access is an internal class method, with no global getter. Existing as_tool and subagent_tool entry points retain their roles. Race and nested-delivery tests obtain live addresses through event.origin; provider smoke tests import the runner module directly.

Accounting is forwarded as plain UsageEvent, without ChildEvent. Existing observer(UsageEvent, ...) and stream.where(UsageEvent) subscriptions receive original local and descendant costs with the same publication identity, kind, provider, and model. Forwarding preserves event_id; an explicit new publication creates a fresh ID and clears the inherited live address. Forwarded usage retains event.origin for addressed budget interventions, while serialization omits live handles. No rollup is generated.

Related issue number

Stack #3525: #3524 (events) → #3523 (variables) → #3526 (routing) → #3527 (documentation) → #3528 (support). No linked issue.

Checks

• I've included any doc changes needed for https://docs.ag2.ai/. See https://docs.ag2.ai/latest/docs/contributor-guide/documentation/ to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

Current implementation: 7884 passed, 9 skipped, 437 deselected, 2 xfailed in the local standard non-LLM suite. The local run deselects four previously reproduced ACP advertised-address networking cases; no repository skips or CI changes were added. GitHub runs the normal workflow matrix.

Focused regression, accounting, telemetry, eval, and cancellation checks: 100 passed, 3 skipped. Mypy passed on the integration layer (255 source files) and with the strict configuration on the stack tip (259 files). Ruff passed for changed Python files. Relative documentation links and Python snippets were checked. Executable code matches the full-suite snapshot; subsequent source edits changed docstrings only.

The original typed-subscription regression was observed failing (only the root's 10 tokens instead of root + child + grandchild costs of 60), then passing for both observer and stream registration. A second regression verifies replay and republication cannot reuse the old live address. CI is rerunning for the updated heads. Website integration/build was not run locally.

AI assistance

Codex assisted with implementation, tests, documentation, and reorganizing the stack. The public surface, source identity, typed subscriptions, persistence/replay, and direct implementation-module imports were verified against the actual implementation. Delivery tests use event-origin handles rather than accessing the registry or selecting recipients by name. No CI workflows or test skips were added.

• I understand the changes in this PR and can explain them in my own words.
• I have verified that the PR description accurately reflects the actual diff.
• If AI assistance was used, I reviewed, tested, and validated the generated code/text before submitting.

[codecov]

Codecov Report

❌ Patch coverage is 92.03980% with 32 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
ag2/tools/subagents/_forwarding.py	92.05%	7 Missing and 10 partials ⚠️
ag2/network/task_mirror.py	7.14%	13 Missing ⚠️
ag2/events/delegation.py	75.00%	1 Missing and 1 partial ⚠️

Files with missing lines	Coverage Δ
ag2/a2ui/dispatch.py	100.00% <100.00%> (ø)
ag2/acp/executor.py	94.17% <100.00%> (-0.06%)	⬇️
ag2/ag_ui/stream.py	95.72% <100.00%> (+0.05%)	⬆️
ag2/agent.py	96.79% <100.00%> (-0.48%)	⬇️
ag2/context.py	97.80% <100.00%> (+0.10%)	⬆️
ag2/eval/sources/_spans.py	94.69% <100.00%> (+1.63%)	⬆️
ag2/eval/trace.py	100.00% <100.00%> (ø)
ag2/events/task_events.py	96.87% <100.00%> (+0.10%)	⬆️
ag2/events/types.py	94.49% <100.00%> (ø)
ag2/history.py	100.00% <100.00%> (ø)
... and 11 more

... and 5 files with indirect coverage changes

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