feat(server): expose `push_sender=` on `create_a2a_server()` so push-notification delivery is reachable

[numarasSigmaSoftware]

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create_a2a_server() accepts push_config_store=, so an agent can persist a buyer's push-notification config — but there is no way to make the SDK actually deliver on it. The underlying handler supports it:

# a2a/server/request_handlers/default_request_handler.py (a2a-sdk 1.0.1)
98:        push_sender: PushNotificationSender | None = None,
125:        self._push_sender = push_sender
340:            self._push_sender          # delivery guarded on it being set

create_a2a_server never passes it and exposes no kwarg for it (grep push_sender adcp/server/ returns no hits), so the parameter is unreachable through the factory. Verified on 6.6.0 and v7.0.0-rc.

Effect: an agent that accepts push-notification configs through the SDK silently never sends anything. Storing a subscription that is never fired is arguably worse than refusing it, since the buyer has no signal that delivery isn't happening.

This matters for AdCP conformance, not just convenience: submitted in the spec's task-status enum is described as long-running work where "push_notification_config is an optional notification channel for terminal completion/failure," and the 3.1.1 notification-config-lifecycle storyboard grades the subscription lifecycle end to end.

Proposed change: add keyword-only push_sender: PushNotificationSender | None = None and pass it through to both DefaultRequestHandler construction sites (a2a_server.py:1182, :1227). Additive; None preserves today's behavior.

Related: this is one of the seams that would let push_config_store= be genuinely useful rather than write-only.

[bokelley]

Triage

Classification: Feature request
Bucket(s): handlers (no handlers label in repo — gap noted)
Status: ready-to-implement

What the experts said:

• code-reviewer: Gap confirmed; non-breaking additive kwarg; PushNotificationSender import path must be verified from installed a2a-sdk==1.0.1 before writing the TYPE_CHECKING import; both DefaultRequestHandler construction sites need test coverage; complementary with feat(server): expose request_handler= on create_a2a_server() #1007 (not subsumed by it).
• dx-expert: Three independent blockers resolved by this fix — silent non-delivery (no log or warning at any layer), asymmetric pair exposure (push_config_store implies push works; push_sender makes it true), no ergonomic workaround short of bypassing the factory entirely; serve() / ServeConfig in serve.py also need threading; add startup warnings.warn() when store is set but sender is absent.

My take: An agent wired with push_config_store but no push_sender advertises push capability in its card, accepts subscriptions, and silently never delivers. Both experts agree the current state is strictly worse than push disabled, and the fix is non-breaking.

Implementation scope:

• src/adcp/server/a2a_server.py: add push_sender: PushNotificationSender | None = None (keyword-only) to create_a2a_server(); thread to both DefaultRequestHandler sites (lines 1182, 1227); verify PushNotificationSender module path from installed a2a-sdk==1.0.1 and add under TYPE_CHECKING alongside existing PushNotificationConfigStore import; add warnings.warn() at line 1159 when push_config_store is not None and push_sender is None.
• src/adcp/server/serve.py: thread push_sender through serve() and ServeConfig so the one-liner entrypoint works end-to-end.
• Docstrings: cross-reference push_config_store ↔ push_sender; note that both must be set for built-in delivery. Update docs/handler-authoring.md push example.
• Tests: cover both constructor paths (callable-URL line 1182, static-URL line 1227), mirroring the existing push_config_store test pattern in tests/test_a2a_server.py.
• _push_supported flag stays as push_config_store is not None; warnings.warn handles the partial-config case. Non-breaking: keyword-only param, None default, identical runtime behavior when omitted.

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