[Tech Debt] Unify sub-agent declarations with generic resource descriptors #1241

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Description

Internal sub-agents currently have a dedicated Java InternalSubagentProvider and a different Python serialized-resource representation. Provider serialization, runtime ownership detection, and setup construction therefore need sub-agent-specific branches. SubagentSetup also extends SerializableResource even though runtime setups are reconstructed from configuration.

Align sub-agent declarations with the existing resource model:

• Keep addResource / add_resource, ordinary ResourceDescriptors, and generic resource providers.
• Provide Java/Python InternalSubagent.builder() helpers that return a descriptor containing the child Agent, description, and input schema. Normalize the direct-Agent registration shorthand at the API boundary.
• Compile the Agent in the plan layer, retain cycle detection and resource-name scopes, and carry the compiled workflow as JSON data under plan.
• Construct InternalSubagentSetup through the generic provider and the normal descriptor/context constructor. Make SubagentSetup a Resource; external sub-agents must be declared with descriptors instead of setup instances.
• Preserve internal execution, nested calls, failure results, and Python task lifecycle forwarding. Cover descriptor metadata and plan JSON in both language directions.

The generic ResourceDescriptor should remain unchanged; child-plan compilation belongs in the plan layer. This refactoring does not add Python-to-Java external sub-agent invocation support or change the JDK requirements for Java internal calls.

Related: #1134. Cross-language external invocation remains tracked separately in #1141.

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