fix: make @poissonians return a Vector like @variables and @brownians

[isaacsas]

Summary

@poissonians returned inconsistent types compared to @variables and @brownians:

Case	@variables / @brownians	@poissonians (before)
Single, inline	[x] (1-element Vector)	bare Num
Multiple, inline	[x, y] (Vector)	(x, y) (Tuple)
Block syntax	[x, y] (Vector)	bare Num or Tuple

This happened because @variables and @brownians delegate to Symbolics.parse_vars (which always returns a Vector via Expr(:vect, ...)), while @poissonians used custom codegen that returned a bare name for single declarations and an Expr(:tuple, ...) for multiple.

Fix

One-line change in _poissonians: replace the conditional bare/tuple return with Expr(:vect, names...) so it always returns a Vector.

No existing tests break since all current usages rely on the side effect of binding variables into scope, not on the return value. New tests verify the return type across all permutations (single/multiple, inline/block).

[isaacsas]

@AayushSabharwal if relevant tests pass this should be good to go. It is a minor change to just be consistent with @variables and always return a vector.