Fix complex value substitution in Complex{Num} expressions

[ChrisRackauckas-Claude]

Summary

Fixes #1109 - Complex number substitution error when substituting complex values into expressions with complex coefficients.

Before this fix:

using Symbolics
@variables x
p = 0.4 + 1.7im*x
substitute(p, Dict(x => 0.2 + 1.0im))
# ERROR: InexactError: Real(0.34 + 1.7im)
# or returns malformed result: 0.4 + (0.34 + 1.7im)*im

After this fix:

using Symbolics
@variables x
p = 0.4 + 1.7im*x
substitute(p, Dict(x => 0.2 + 1.0im))
# Returns: -1.2999999999999998 + 0.34im  (correct!)

Changes

• Modified (s::SymbolicUtils.Substituter)(x::Complex{Num}) in src/complex.jl to properly handle the case where substituted values are complex constants
• When both the real and imaginary parts become constant after substitution, properly compute the complex result using standard complex arithmetic: (a + b*im) where a and b can themselves be complex
• Added comprehensive tests in test/complex.jl for complex value substitution

Root Cause

The previous implementation applied substitution to real and imaginary parts separately:

Complex{Num}(s(real(x)), s(imag(x)))

This works fine for real substitutions, but when substituting a complex value like 0.2 + 1.0im into 1.7im*x:

• The imaginary part 1.7*x becomes 0.34 + 1.7im (a complex constant wrapped in Num)
• Constructing Complex{Num}(0.4, 0.34+1.7im) creates an invalid structure

The fix detects when both parts are constants and properly evaluates the complex expression.

Test plan

• Added test case from issue Error with substitution of a Complex{Float64} value in polynomial with complex coefficients #1109
• Added tests for expressions where both real and imag parts contain variables
• Added regression test for real value substitution
• Added test for two-variable complex substitution
• Added tests that Symbolics.value, simplify, and expand work on results
• All existing test/complex.jl tests pass
• CI tests

🤖 Generated with Claude Code

[ChrisRackauckas]

Looks pretty reasonable to me.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 86.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.81%. Comparing base (9df21d7) to head (389dd6c).
⚠️ Report is 9 commits behind head on master.

Files with missing lines	Patch %	Lines
src/complex.jl	86.66%	2 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1710      +/-   ##
==========================================
+ Coverage   80.10%   80.81%   +0.70%     
==========================================
  Files          55       55              
  Lines        5187     5196       +9     
==========================================
+ Hits         4155     4199      +44     
+ Misses       1032      997      -35     

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