🧹 Refactor compute_pagerank from compute_and_store

[LeandroPG19]

🎯 What: The code health issue addressed
Extracted the core compute_pagerank algorithm into its own pure function in rust/src/graph/pagerank.rs, removing it from the compute_and_store function.

💡 Why: How this improves maintainability
The compute_and_store function was a long, complex function mixing DB I/O and graph computation logic. By extracting the pure calculation, it makes the function more readable and the calculation independently testable.

✅ Verification: How you confirmed the change is safe
Ran cargo test and added explicit unit tests for the extracted compute_pagerank calculation for empty, simple (cycle), and dangling node inputs. All passed. Also ran cargo fmt and cargo clippy.

✨ Result: The improvement achieved
A cleaner, more maintainable file with independent testing of the PageRank algorithm.

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PR created automatically by Jules for task 17353326867845944981 started by @LeandroPG19

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