Fix multi-seed ego_graph offset handling

[sauravsingla]

Summary

This draft addresses #4191.

The lower-level pylibcugraph.ego_graph API already returns subgraph offsets for
multiple seed vertices, but the high-level Python wrapper currently discards
them.

This proposal:

• preserves existing single-seed behavior;
• prevents ambiguous composite output for multiple seeds;
• exposes offsets through return_offsets=True;
• adds regression tests covering multi-seed behavior;
• adds weighted graph and renumbered graph coverage.

Testing

Completed without GPU:

• Python syntax validation
• py_compile
• mocked wrapper control-flow tests
• input validation tests

GPU validation is still required for:

• cuDF integration
• pylibcugraph execution
• weighted graph behavior
• renumbered graph behavior
• SG/MG CI

This is opened as a draft to discuss the preferred public API before finalizing the implementation.

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[sauravsingla]

This PR was closed accidentally while I was trying to convert it to draft. The branch and commits are still available.

Could a maintainer please reopen this PR, or advise whether I should open a new draft PR from the same branch?

The change addresses #4191 and is intended for API discussion and GPU CI validation before being marked ready for review.

[alexbarghi-nv]

/ok to test

[copy-pr-bot]

/ok to test

@alexbarghi-nv, there was an error processing your request: E1

See the following link for more information: https://docs.gha-runners.nvidia.com/cpr/e/1/

[alexbarghi-nv]

/ok to test 62e035b

[sauravsingla]

Hi @alexbarghi-nv, just checking whether the CI run for commit 62e035b needs to be retriggered. Please let me know if anything is needed from my side.