Fix / in step labels breaking source reference parsing

[jmchilton]

Summary

• Add resolve_source_reference() helper in model.py that matches known labels (longest first) before falling back to split("/"), handling labels like Host/Contaminant Reference Genome
• Update converter.py (BaseConversionContext.step_output()), normalize.py (_replace_anonymous_output_references, _ensure_implicit_step_outs), and cytoscape.py (to_cytoscape) to use the new helper
• Add unit tests for the helper and integration/round-trip tests with / in labels

Closes #145

Test plan

• All 101 tests pass (88 existing + 13 new)
• Unit tests for resolve_source_reference cover: simple labels, slash-in-label with/without output, multiple slashes, longest-match ordering, fallback behavior
• Integration tests verify to-native conversion and full round-trip with / in input labels, step labels, and chained connections

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 41.03%. Comparing base (321bfaa) to head (5a029d5).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #148   +/-   ##
=======================================
  Coverage   41.03%   41.03%           
=======================================
  Files          51       51           
  Lines        1974     1974           
  Branches      441      441           
=======================================
  Hits          810      810           
  Misses       1047     1047           
  Partials      117      117           

Flag	Coverage Δ
unittests	41.03% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.