fix: small-corpus path

[KartikVashishta]

Description

Tiny corpora + CacheType.memory crashed with
ValueError: Columns must be same length as key (and follow-on dtype issues).

This PR hardens three spots:

1. build_noun_graph._extract_edges

   • Expand edges column into source/target via pd.DataFrame, padding bad rows and dropping NaNs leading to no broadcast error.

2. graph_to_dataframes

   • Store numpy embeddings as list so the single-column assignment can’t broadcast-fail.

3. prune_graph

   • a) Return early for empty graphs.
   • b) astype(str) on source/target before merges to avoid float64 vs object mismatch.

Related Issues

Closes #1983

Proposed Changes

• graphrag/index/operations/build_noun_graph/build_noun_graph.py
• graphrag/index/operations/graph_to_dataframes.py
• graphrag/index/operations/prune_graph.py
• New test tests/unit/indexing/graph/test_small_corpus_bug.py

Checklist

• [x] Tested locally on a one-line corpus (input/tiny.txt) with IndexingMethod.Fast & CacheType.memory.
• [x] I have reviewed the code changes.
• [x] I have updated the documentation (if necessary).
• [x] I have added appropriate unit tests (if applicable).

Additional Notes

These fixes touch only the small-corpus edge-cases - normal pipelines are unaffected. Heads-up: the prompt files have been altered, can be discarded. Shout if you’d like them dropped or squashed.

[KartikVashishta]

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