[FEATURE] Complete public cross-AgentInstance A2A delegation (K11)

[EItanya]

Summary

Complete K11 by allowing one independently owned AgentInstance to invoke another through the public A2A gateway.

Scope

• Check that the target AgentInstance is ready and not already busy.
• Issue short-lived credentials scoped to the exact target and delegated invocation.
• Persist trusted parent/root task lineage.
• Enforce delegation depth, deadlines, and cycle detection.
• Stream target task events through the caller while retaining a normal public task and history on the target.
• Reject lineage or authorization metadata supplied by an untrusted runtime.
• Route any convenience MCP invocation through the same public A2A path.
• Add authorization, failure, cancellation, streaming, and multi-hop E2E coverage.

Approval and input_required continuation already use the public A2A semantics and should be reused rather than reimplemented.

Out of scope

• Provisioning private child Actors from Dedicated template bindings.
• Atomic lifecycle, checkpoint, or fork operations over multi-member instances.

Those belong to K9 Dedicated topology.

Completion criteria

• Every accepted delegation creates an ordinary durable task on the target AgentInstance.
• Credentials cannot address another target or outlive the delegated operation.
• Cycles, excessive depth, unauthorized targets, and busy targets are rejected deterministically.
• Streaming, cancellation, deadlines, lineage, and recovery are covered by E2E tests.

Roadmap: docs/plans/api-v2-execution-plan.md, K11.

[yashrajshuklaaa]

@EItanya hi i would love to work on this issue. could you please assign it to me?