[api][plan][runtime] Introduce AGENT resource type and sub-agent invocation API

[pltbkd]

Design discussion: #909

Purpose of change

Introduces AGENT as a first-class resource type and the caller-facing sub-agent invocation API across Java / Python / YAML, per discussion #909.

SubagentSetup is the general sub-agent framework. This PR lands the framework and the one way to plug into it today: a custom SubagentSetup implementation. Registering an Agent directly as an internal sub-agent is a follow-up addition.

Included:

• ResourceType.AGENT; register via addResource(name, AGENT, setup) and via YAML subagents:.
• Subagent (caller interface) + SubagentSetup (base) + Result + BaseSubagentCallable (captures failures into Result).
• call() / asAsyncCallable() run through durable execution; a deterministic (sessionId, callId) identity lets failover replay reuse cached results instead of re-invoking. Use asAsyncCallable() instead of callAsync() to support batch async execution.
• sessionId is framework-generated and deterministic; callers may pass their own to continue a session. callId is auto-generated from the per-session conversation ordinal.

Evolved from the #909 proposal during review:

• Result carries a serializable errorMessage (the failure's full stack trace) instead of a live Exception, so it survives durable persistence.
• The durable id is derived solely from (sessionId, callId).

Notes

• Follow-up additions: ChatModel calling sub-agents (sub-agent-as-tool dispatch), and internal sub-agent (registering an Agent directly as an AGENT resource, compiled into a scoped child plan, and its isolated execution).
• Cross-language execution (an action calling a sub-agent implemented in the other language) is not supported yet; registration round-trips across the plan-JSON boundary, execution will be added later.
• Timeout / cancellation semantics are not included yet; planned as a follow-up.
• Result and the parallel async primitive (executeAllAsync) overlap with Parallel Tool Execution ([Feature] Parallel Tool Call Execution #926) and can be aligned/iterated during review.

Tests

• Java unit: AGENT registration, plan compilation (SubagentSetup + YAML descriptor), and deterministic-id context / failover-recovery / operator-integration tests.
• Python unit: mirrors the Java surface (registration, plan resources, id delegation).
• e2e: ExternalSubagentTest — programmatic and YAML-declared sub-agent invocation (including failure surfaced via Result) on a real Flink job.

API

Additive public API, kept semantically aligned across Java / Python / YAML: ResourceType.AGENT, Subagent, SubagentSetup, Result, BaseSubagentCallable, RunnerContext#nextSessionId() / nextCallId(sessionId), and the YAML subagents: block.

No breaking changes to existing APIs.

Documentation

• doc-needed but deferred: add once the internal sub-agent lands and the API stabilizes
• doc-not-needed
• doc-included