Fix Documentation build: CanonicalMoments SymbolicsExt precompile failure on Symbolics 7

[ChrisRackauckas-Claude]

Problem

The Documentation build fails before makedocs is reached, while precompiling CanonicalMoments → SymbolicsExt:

WARNING: Imported binding Symbolics.array_term was undeclared at import time during import to SymbolicsExt.
ERROR: LoadError: FieldError: type Union has no field `parameters`, available fields: `a`, `b`
  [2] (::Symbolics.var"#is_wrapped_array_eltype#wrap_func_expr##9")(T::Type)
    @ Symbolics .../src/wrapper-types.jl:220

That failure cascades into Failed to precompile OUQBase, so docs/make.jl dies at its first using.

Root cause

#40 widened the CanonicalMoments compat entry to Symbolics = "6.29.2, 7.30". OUQBase also allows ModelingToolkit = "9.69, 11.31", so the docs environment now resolves Symbolics 7 (7.36.0 / MTK 11.40.0 locally). Two things in lib/CanonicalMoments/ext/SymbolicsExt.jl do not survive that:

1. @register_array_symbolic introspects every declared argument type. AbstractVecOrMat{<:Real} is a UnionAll whose body is a Union, and Symbolics 7's is_wrapped_array_eltype unwraps the UnionAll and then reads .parameters, which a Union does not have. Hence the FieldError.
2. Symbolics.array_term was removed in Symbolics 7 (hence the undeclared-binding warning), so the hand-rolled companion matrix term in _companion_matrix cannot be constructed.

Fix

1. Declare the second argument of the EigvecWeightAlg registration as AbstractArray{<:Real}. This still covers the vector and matrix cases that AbstractVecOrMat was there for, but is not a Union, so the macro can introspect it. (Registering AbstractVector and AbstractMatrix separately is not an option: both expand to the same (AbstractMatrix, BasicSymbolic) method and trip "Method overwriting is not permitted during Module precompilation".)
2. Drop the manual array_term construction and instead register CanonicalMoments._companion_matrix itself with @register_array_symbolic. The macro builds the lazy array term for symbolic inputs and falls through to the existing numeric SymTridiagonal(-B, sqrt.(C[2:end])) implementation otherwise, which is what the old code emulated by hand. This works unchanged on Symbolics 6 and 7, so no compat entry has to be narrowed.

The now-unused unwrap/wrap/array_term imports and the LinearAlgebra dependency of the extension are removed along with the code that used them.

Verification

• docs/make.jl runs to completion locally (Julia 1.12.4, Symbolics 7.36.0, ModelingToolkit 11.40.0, SymbolicUtils 4.45.0), with only the pre-existing missing_docs warnings that warnonly already tolerates.
• The extension precompiles and the symbolic path round-trips against the numeric implementation on both Symbolics 6.29.2 and 7.36.0:

@variables B[1:3] C[1:3]
A = CanonicalMoments._companion_matrix(collect(B), collect(C))   # Symbolics.Arr{Num, 2}
vals = EigvalSupportAlg()(A)                                     # Symbolics.Arr{Num, 1}
vecs = EigvecWeightAlg()(A, vals)                                # Symbolics.Arr{Num, 2}
f = Symbolics.build_function(vals, collect(B), collect(C); expression = Val(false))[1]
f([1.0, 2.0, 3.0], [1.0, 4.0, 9.0]) ≈
    EigvalSupportAlg()(CanonicalMoments._companion_matrix([1.0, 2.0, 3.0], [1.0, 4.0, 9.0]))  # true

• Runic reports no formatting diff for the changed file.

Out of scope, but worth flagging

OUQBase's Core tests still error on Symbolics 7, for an unrelated reason in lib/OUQBase/src/operators.jl:

MethodError: promote_symtype(::OUQBase.𝔼_, ::Type{Real}) is ambiguous.
Candidates:
  promote_symtype(::OUQBase.𝔼_, x)                              @ OUQBase src/operators.jl:11
  promote_symtype(::SymbolicUtils.Operator, ::Type{T}) where T   @ SymbolicUtils

SymbolicUtils 4 added the ::Type{T} method, so promote_symtype(::𝔼_, x) needs to become promote_symtype(::𝔼_, ::Type{T}) where {T}. That code is not reached by the docs build, so I left it out of this PR to keep the change limited to the Documentation failure.

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