Fix usage of internal @check macro

[nickrobinson251]

Fixes an undefvar error (found by JET.jl on a package that calls CSV.write(file::String, table::NamedTuple):

│││││││││││┌ writebom(buf::Vector{UInt8}, pos::Any, len::Int64) @ CSV ~/.julia/packages/CSV/XLcqT/src/write.jl:306
││││││││││││ `CSV.io` is not defined: CSV.io

Repro:

julia> using CSV; CSV.writebom(UInt8[1,2,3], 2, 1)
ERROR: UndefVarError: `io` not defined
Stacktrace:
 [1] writebom(buf::Vector{UInt8}, pos::Int64, len::Int64)
   @ CSV ~/repos/CSV.jl/src/write.jl:311
 [2] top-level scope
   @ REPL[2]:1

the @check macro assumes certain variables are in scope, namely io (and buf, pos, len), but that wasn't true for writebom.

this PR updates writebom to take a variable named io, and updates @check to assert a io variable is defined.
the other variables are always referenced in the expanded code, so we don't @assert @isdefined those.

[Copilot]

Pull request overview

This PR fixes an UndefVarError in the writebom function caused by the internal @check macro assuming an io variable is in scope when it wasn't. The fix updates writebom to accept an io parameter and adds a compile-time assertion to the @check macro to catch similar issues in the future.

Changes:

• Updated writebom function signature to include io parameter
• Added assertion in @check macro to verify io is defined in caller's scope
• Updated all writebom call sites to pass the io argument
• Added test case that exercises the fixed code path

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/write.jl	Updated writebom signature to accept io parameter, added assertion to @check macro, and updated all call sites
test/write.jl	Added test case for writebom function that exercises the bug fix

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.65%. Comparing base (7819c85) to head (2fe45af).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1179      +/-   ##
==========================================
+ Coverage   90.56%   90.65%   +0.09%     
==========================================
  Files           9        9              
  Lines        2321     2322       +1     
==========================================
+ Hits         2102     2105       +3     
+ Misses        219      217       -2     

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