be smarter when assembling the constraints expression

[cdiener]

• fix [BUG] Changing from hyrid to glpk is incredibly slow cobrapy#1473
• description of feature/fix
• tests added/passed
• add an entry to the next release

When assembling Constraint.expression the matrix interface would iterate over all variables for each constraint. Now it prefilters the coefficients and only uses the relevant variables making the expression sparse as it should be.

Also fixed a typo.

[Copilot]

Pull request overview

This PR optimizes how Constraint.expression is assembled in the matrix-based interfaces (used by the hybrid solver), avoiding per-constraint iteration over every model variable by instead constructing expressions only from non-zero stored coefficients. It also fixes a local typo in the objective parsing code path.

Changes:

• Build Constraint.expression from the sparse (constraint, variable) -> coefficient mapping instead of scanning all variables.
• Fix typo quadratic_coeffients → quadratic_coefficients in the objective setter.

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[oxinabox]

This makes sense to me.

I can't test it.
Because versioneer on this branch thinks it is v2.dev
and so pip refuses to install the branch.
(If this isn't merged/tagged by time i start work tomorrow, I will hack around it, and test it.)