fix(core): fix nested subgraph rendering for 3+ level nesting in draw_mermaid

[Taehan (Hank) Kim (taehan79-kim)]

Fixes #35492

The "empty subgraphs" section in draw_mermaid() skips multi-level prefixes (e.g., parent:child) due to the if ":" not in prefix check, causing 3+ level nested subgraph boxes to not render at all. This fix splits the prefix on : and opens subgraph blocks for each nesting level.

Before: if ":" not in prefix → skips multi-level prefixes entirely
After: prefix.split(":") → iterates each level, opens subgraph blocks per level

How did you verify your code works?

• All 22 tests in test_graph.py pass (18 snapshots passed) (17 snapshots passed, no updates needed)
• Added test_nested_empty_subgraph_mermaid regression test
• make format and make lint pass
• Manually verified correct rendering for 3-level, 4-level nesting with draw_mermaid_png()

Screenshots

Before (bug):

After (fix):

[codspeed-hq]

Merging this PR will not alter performance

✅ 13 untouched benchmarks
⏩ 2 skipped benchmarks¹

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_{Comparing taehan79-kim:taehan79-kim/fix-nested-subgraph-rendering (a888432) with master (1338871)²}

Footnotes

1. 2 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

2. No successful run was found on master (a79298a) during the generation of this report, so 1338871 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[Taehan (Hank) Kim (taehan79-kim)]

Friendly ping Eugene Yurtsev (@eyurtsev), this has been open for ~3 weeks now. This was the first PR submitted for this issue, all tests pass, and it's a small, focused change. Happy to address any feedback if needed!