fix(sdk): preserve context schema typing in create_deep_agent

[LRRuan]

Summary

• preserve the runtime context type in create_deep_agent's context_schema signature
• return a parameterized CompiledStateGraph so type checkers infer invoke(..., context=...) correctly
• keep runtime behavior unchanged

Testing

• python3 -m py_compile libs/deepagents/deepagents/graph.py

Closes #1778

[github-actions]

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