fix(flow): propagate nested FlowNode input requests

[oracle26ashish]

Summary

• add FlowNode propagate_pending_input semantics with default propagation and version gating
• propagate nested LangGraph interrupts through parent FlowNodes and fail explicitly when disabled
• update Python/TypeScript tests, docs, changelog, and generated schema snapshots

Tests

• env SKIP_LLM_TESTS=1 PYTEST_ADDOPTS=--assert=plain uv run --python 3.11 --with-editable .[langgraph] --with cryptography --with pytest==9.0.3 --with pytest-asyncio==1.3.0 pytest -q tests/adapters/langgraph/flows/test_flownode.py tests/adapters/langgraph/flows/test_inputmessagenode.py
• uv run --python 3.11 --with-editable . --with pytest==9.0.3 --with pytest-asyncio==1.3.0 pytest -q tests/serialization/test_flow_node.py tests/validation/io_validation/test_flownode_iosvalidation.py tests/test_schema_generation.py::test_component_schema_corresponds_to_agentspec_schema_in_docs
• npm test -- tests/flows/nodes.test.ts tests/serialization/version-gates.test.ts
• npm run lint
• git diff --check

Addresses #183

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