Handle identity-wrapped constants in Maxima serializer

[Amin-El-Sayed]

Fixes a Maxima serialization failure for expressions where Symbolics wraps constants such as pi/π in identity(...).

Example fixed:

using Symbolics, SymbolicIntegration, SymbolicIntegrationMaxima
@variables x L
m = MaximaMethod(timeout=10)
integrate(sin(π * x / L)^2, x, 0, L, m; assumptions=(L > 0,))
# (1//2)*L

Changes:

• serialize identity(arg) as arg for Maxima input
• add regression coverage for π * x
• add regression coverage for ∫_0^L sin(πx/L)^2 dx = L/2 under L > 0

Validation:

TEST_GROUP=basic julia --startup-file=no --project=lib/SymbolicIntegrationMaxima -e 'import Pkg; Pkg.test()'
SymbolicIntegrationMaxima.jl | 39/39 passed

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 54.51%. Comparing base (8647f0d) to head (1eb7ed7).
⚠️ Report is 3 commits behind head on main.
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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #120      +/-   ##
==========================================
+ Coverage   50.98%   54.51%   +3.52%     
==========================================
  Files          23       23              
  Lines        4309     4309              
==========================================
+ Hits         2197     2349     +152     
+ Misses       2112     1960     -152     

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[Amin-El-Sayed]

I also bumped the SymbolicIntegrationMaxima subpackage version to 0.1.2 in this PR.

Reason: after this is merged, the correct registry path is to register a new SymbolicIntegrationMaxima version from lib/SymbolicIntegrationMaxima in this repository. The previous General registry PR attempted to update the tree hash of already-registered v0.1.1, which is not allowed, so I closed it. Existing v0.1.1 should remain unchanged; v0.1.2 can be the first version registered from the monorepo subdir.

[Amin-El-Sayed]

Follow-up pushed: the remaining basic CI failures were order-sensitive serialization tests. Symbolics/SymbolicUtils may reorder commutative sums/products across Julia/dependency versions, e.g. %pi*x vs x*%pi or asinh(x)+atanh(x) vs the reverse. The new tests check that the required Maxima tokens/functions are present instead of requiring one textual order.\n\nLocal check after the change:\n\n\nTEST_GROUP=basic julia --startup-file=no --project=lib/SymbolicIntegrationMaxima -e 'using Pkg; Pkg.test()'\n# SymbolicIntegrationMaxima.jl | 41 passed\n