fix(backend): Include sub-graphs in graph-level credentials support

[Pwuts]

The Library Agent credentials UX (#9789) currently doesn't work for sub-graphs.

Changes 🏗️

• Include sub-graphs in generating Graph.credentials_input_schema
• Propagate node_credentials_input_map into AgentExecutionBlock executions

Checklist 📋

For code changes:

• [x] I have clearly listed my changes in the PR description
• [x] I have made a test plan
• [ ] I have tested my changes according to the test plan:
  • Import a graph with sub-graphs that need credentials
  • Run this agent from the Library
  • [ ] -> Should work

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