Add recursive process graph unrolling

[Silvren]

Summary

• add Connection.unroll_process_graph() to turn workflows with UDP references into self-contained flat process graphs
• resolve registered UDPs from /process_graphs/{id} and HTTP(S)-namespaced remote process definitions
• recursively inline nested UDPs, including definitions used inside callback process graphs
• bind invocation arguments and process defaults while preserving callback parameter scopes
• rewrite node references and generate collision-safe node IDs
• detect recursive definitions, missing required arguments, invalid definitions, and ambiguous remote listings
• cache process listings and resolved definitions during an unroll operation
• keep the input graph unchanged

Testing

• python -m pytest tests/internal tests/rest/test_udp.py -q (205 passed)
• python -m pytest tests/internal/test_graph_unrolling.py tests/rest/test_connection.py::test_unroll_process_graph_registered_udp tests/rest/test_connection.py::test_unroll_process_graph_remote_definition -q (7 passed)
• pre-commit run --files CHANGELOG.md openeo/internal/graph_unrolling.py openeo/rest/connection.py tests/internal/test_graph_unrolling.py tests/rest/test_connection.py

Fixes #749