Fix event equality by excluding zero_crossing_id from comparison

[ChrisRackauckas-Claude]

Summary

This PR fixes issue #3907 where systems with identical continuous events were no longer considered equal after the v10 update.

The root causes were:

1. zero_crossing_id was generated via gensym() for each callback, making otherwise identical callbacks unequal
2. AffectSystem equality used isequal for its internal System, but System uses reference equality (===)

Changes:

• Exclude zero_crossing_id from AbstractCallback equality and hash (it's an internal identifier, not semantically meaningful for equality)
• Use isapprox instead of isequal for system comparison in AffectSystem equality (since System has a proper isapprox implementation for structural comparison)
• Hash AffectSystem based on semantic content (equations, observed) rather than system identity

Before this fix:

using ModelingToolkit
using ModelingToolkit: t_nounits as t, D_nounits as D
@parameters p d
@variables X(t)
eqs = [D(X) ~ p - d*X]
continuous_events = [X ~ 1] => [X ~ Pre(X) + 1.0]
@mtkcompile sys1 = System(eqs, t; continuous_events)
@mtkcompile sys2 = System(eqs, t; continuous_events)

# Events were not equal even though they're semantically identical
e1 = ModelingToolkit.continuous_events(sys1)
e2 = ModelingToolkit.continuous_events(sys2)
isequal(e1[1], e2[1])  # false

After this fix:

isequal(e1[1], e2[1])  # true
hash(e1[1]) == hash(e2[1])  # true

# Systems with same name are now approximately equal
sys3 = System(eqs, t; continuous_events, name=:test)
sys4 = System(eqs, t; continuous_events, name=:test)
sys3 = mtkcompile(sys3)
sys4 = mtkcompile(sys4)
isapprox(sys3, sys4)  # true

Test plan

• Verified the fix resolves the issue from Identical systems that contain events are no longer equal #3907
• Verified events with identical definitions are now equal
• Verified event hashes are consistent for identical events
• Verified systems with same name and identical events are approximately equal via isapprox

Fixes #3907

🤖 Generated with Claude Code