Feature/defensive checks and tests

[FBumann]

Closes # (if applicable).

Changes proposed in this Pull Request

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Unit tests for new features were added (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.

Summary by CodeRabbit

• New Features

  • Added optional section header control for quadratic constraints file output.

• Bug Fixes

  • Fixed quadratic constraint label validation and duplicate detection.
  • Corrected MOSEK quadratic coefficient handling to match solver conventions.
  • Eliminated redundant assignment in solver output processing.

• Tests

  • Expanded quadratic constraint solver testing across multiple solvers and I/O APIs with new integration test fixtures.

• Documentation

  • Enhanced quadratic matrix construction documentation and notebook examples.

_{✏️ Tip: You can customize this high-level summary in your review settings.}

[coderabbitai]

Caution

Review failed

The pull request is closed.

Walkthrough

This PR enhances quadratic constraint handling through validation additions, testing infrastructure improvements, and bug fixes. It includes duplicate QC label validation, Solution representation updates, file export enhancements, MOSEK convention alignment, and a comprehensive test suite for multi-solver QC solving.

Changes

Cohort / File(s)	Summary
Quadratic Constraint Validation linopy/matrices.py	Added runtime validation to detect duplicate QC labels globally; enhanced Qc docstring with construction notes; introduced skipped\_rows counter in qc_linear to catch unmapped linear terms and report index mismatches.
Solution Representation linopy/constants.py	Modified Solution __repr__ to compute and display qc\_dual count from self.solution.qc_dual when present, appending it after primal count with conditional suffix.
File I/O and Export linopy/io.py	Added optional write_section_header parameter to quadratic_constraints_to_file; introduced label validation raising ValueError for missing metadata; updated MOSEK QC handling to pass full Q data without division by 2, aligning with MOSEK convention.
Solver Logic linopy/solvers.py	Fixed duplicate variable assignment: solution = solution = maybe_adjust_objective_sign(...) → solution = maybe_adjust_objective_sign(...).
Testing Infrastructure test/test_quadratic_constraint.py	Added QC solver availability detection with configurable qc_solver_params; introduced six pytest fixtures (qc_circle_model, qc_multidim_model, qc_mixed_model, qc_cross_terms_model, qc_geq_model, qc_equality_model) and new TestQuadraticConstraintSolving class with parameterized solver tests covering constraint satisfaction and objective validation across multiple QC types and solvers.
Documentation examples/quadratic-constraints.ipynb	Reformatted cell sources from single strings to multi-line arrays; removed prior execution outputs and metadata, preparing notebook for fresh execution.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

• linopy/io.py: MOSEK QC convention change (no division by 2) requires verification against MOSEK API expectations and testing coverage.
• test/test_quadratic_constraint.py: Large test suite addition; verify fixture models are correctly constructed and assertions appropriately cover solver edge cases and nonconvexity handling.
• linopy/matrices.py: Duplicate label validation logic; ensure all code paths populate labels before checks and error messages are actionable.
• linopy/solvers.py: Bug fix is straightforward but verify maybe_adjust_objective_sign side effects and return value handling.

Possibly related PRs

• Feature/dual-value-retrieval #1: Overlapping changes to QC metadata handling, Solution.qc\_dual representation, and IO/MOSEK export conventions.
• Feature/centralize quadratic solvers check #3: Concurrent modifications to quadratic constraint machinery including io.py MOSEK handling, matrices.py QC assembly, and Solution qc\_dual representation.
• Feature/mosek direct api #2: Related QC handling updates across io.py MOSEK conventions, matrices.py validation, and Solution representation/usage.

Poem

🐰 Quadratic constraints now leap with care,
Duplicate labels caught mid-air!
MOSEK conventions aligned just right,
Tests bloom in parametric light. ✨
Validation guards each QC's flight!

✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch feature/defensive-checks-and-tests

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📜 Recent review details

Configuration used: CodeRabbit UI

Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between c5c3914 and 206c768.

📒 Files selected for processing (6)

• examples/quadratic-constraints.ipynb (9 hunks)
• linopy/constants.py (1 hunks)
• linopy/io.py (4 hunks)
• linopy/matrices.py (5 hunks)
• linopy/solvers.py (1 hunks)
• test/test_quadratic_constraint.py (2 hunks)

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