fix: repair FRLM VMT coverage fallback

[samay2504]

This PR fixes issue #520 by correcting the fallback path in FRLM.get_vmt_coverage() so it initialises missing flow coverage state and delegates to the existing get_flow_coverage() method rather than calling a non-existent method, and it adds a regression test that removes flow_coverage before invocation to confirm the method now returns VMT metrics instead of raising AttributeError.

[Copilot]

Pull request overview

This PR fixes FRLM’s VMT coverage fallback path so get_vmt_coverage() no longer calls a non-existent flow coverage method when flow_coverage has been removed, and adds a regression test to prevent the AttributeError reported in issue #520.

Changes:

• Update FRLM.get_vmt_coverage() to initialize missing flow_coverage state and delegate to the existing get_flow_coverage() implementation.
• Add a regression test that deletes flow_coverage before calling get_vmt_coverage() and asserts VMT metrics are returned.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
spopt/locate/flow.py	Fixes the fallback in get_vmt_coverage() by initializing flow_coverage and calling get_flow_coverage() instead of a missing method.
spopt/tests/test_locate/test_flow.py	Adds a regression test covering the missing-flow_coverage attribute scenario from issue #520.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.0%. Comparing base (9b36063) to head (5edad42).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #521     +/-   ##
=======================================
+ Coverage   76.9%   77.0%   +0.1%     
=======================================
  Files         29      29             
  Lines       3700    3701      +1     
=======================================
+ Hits        2846    2849      +3     
+ Misses       854     852      -2     

Files with missing lines	Coverage Δ
spopt/locate/flow.py	73.4% <100.0%> (+0.2%)	⬆️

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