Update ParameterGradientKeys for vsite key matching

[lilyminium]

Description

• Fixes Can't re-fit virtual sites due to inability to identify them uniquely #726
• updates ParameterGradientKeys to expect all required info for matching vsite keys
• Requires matching ForceBalance update

note -- this may not be the best way to do it, i'm not sure if we can integrate canonical Interchange PotentialKeys instead?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.21%. Comparing base (65fc593) to head (52fc6d1).
⚠️ Report is 1 commits behind head on main.

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