Add latexify hooks for metadata and ops

[oameye]

Summary

Add extension points to the Latexify pipeline so downstream packages can customize LaTeX rendering based on:

1. metadata attached to BasicSymbolic nodes, and
2. the operator of a call.

This enables non-pirating, opt-in rendering for DSLs (e.g., angle-bracket averages, sum prefixes) without forking Symbolics' LaTeX code.

What changed

• Introduce _toexpr_metadata(O, ::Type{Ctx}, val; latexwrapper) to allow metadata-driven LaTeX overrides.
• Introduce _toexpr_op(op, args; latexwrapper) to allow operator-driven LaTeX overrides.
• Add _toexpr_plain so hooks can fall back to the default LaTeX formatting.

Example usage

function LatexifyExt._toexpr_op(::typeof(avg), args; latexwrapper=LatexifyExt.default_latex_wrapper)
    inner = LatexifyExt._toexpr_plain(args[1]; latexwrapper)
    inner_s = strip(latexify(inner).s, '$')
    return LaTeXString("\\langle " * inner_s * " \\rangle")
end

function LatexifyExt._toexpr_metadata(O, ::Type{SumIndices}, val; latexwrapper=LatexifyExt.default_latex_wrapper)
    # return a LaTeX AST or LaTeXString that prefixes a sum, e.g. "\\Sigma( ... )"
end

Example output

• avg(x + y) renders as \langle x + y \rangle
• A sum metadata hook can render as \Sigma(i=1:N) ...

Tests

• Added a test in test/latexify.jl showing a custom op hook returning a LaTeXString.

Motivation

Downstream packages currently have no hook to customize LaTeX output for custom operators or metadata-rich nodes (e.g., average brackets, sum prefixes). This PR adds general hooks without changing default behavior.

[oameye]

@AayushSabharwal Can also you have a look at this PR? :)

[AayushSabharwal]

Is defining methods of functions in extensions like this valid? I thought Base.get_extension was internal and only for debugging?

[oameye]

Yeah it is heavily discouraged by the Julia Base maintainers. Never really understood why.

The real issue is that the hookable functions live in the extension, so downstream packages have no clean way to extend them. We can move _toexpr_metadata and _toexpr_op to the main Symbolics module with nothing-returning fallback methods. The extension then calls those Symbolics-defined functions. Downstream packages can extend them directly via import Symbolics without touching Base.get_extension. _toexpr_plain and default_latex_wrapper are Latexify-coupled helpers and stay in the extension; they'd only be needed for composing output inside a hook, and at that point the user already has Latexify loaded.

[oameye]

Do you think this is the correct solution?