[msardara]

Closes #383. Stacked on #397.

Add live exchanges and a shared execution/input/response runtime across send modes, with owned input receivers, execution IDs, cancellation, and queued webhook delivery. Wire all four transports and advertise protocol 1.1 after client negotiation lands; HTTP exchange routing uses the SDK-specific x-a2a-rs-live-exchange header.

The preceding PR contains the existing handler module extraction. Working-task input joins and executor-selected task creation follow in #390 and #392.

Validation: 1,043 workspace tests and doctests passed (excluding the external ITK agent), plus all-target Clippy.

[codecov]

Codecov Report

❌ Patch coverage is 93.83117% with 152 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
a2a-server/src/handler/execution.rs	77.30%	32 Missing ⚠️
a2a-server/src/handler/turns.rs	82.94%	22 Missing ⚠️
a2a-server/src/executor/inputs.rs	92.21%	19 Missing ⚠️
a2a-server/src/handler/response.rs	96.40%	16 Missing ⚠️
a2a-server/src/handler/exchange.rs	96.26%	8 Missing ⚠️
a2a-server/src/handler/uplink.rs	95.37%	8 Missing ⚠️
a2a-server/src/handler/push_delivery.rs	96.50%	7 Missing ⚠️
a2a-server/src/handler/output.rs	89.65%	6 Missing ⚠️
a2a-server/src/handler/protocol.rs	90.32%	6 Missing ⚠️
a2a-server/src/handler/subscription.rs	91.37%	5 Missing ⚠️
... and 9 more

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