Fix chained kwarg scoping after positional args

[yoshibase]

Summary

Fixes #379

Stop parsing keyword flags for the current command once a positional argument is encountered, so chained commands receive their own flags.

Test plan

• python3 -m unittest fire.fire_test.FireTest.testChainedKwargsWithSameParameterName

Made with Cursor

[google-cla]

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[yoshibase]

Pausing this PR for now: the cla/google check requires signing Google's CLA, and our contributor policy currently excludes repositories that require CLA signing. The fix remains prepared locally and can be resubmitted once CLA signing is feasible. Thank you for understanding.