Add PLEXOS custom-constraint extraction script

[nick-gorman]

Adds scripts/extract_plexos_constraints.py, which extracts the custom constraint definitions ISPyPSA needs — the REZ export-limit constraints and one gas-generation constraint — from an AEMO PLEXOS XML model into intermediate CSVs under src/ispypsa/templater/plexos/<version>/.

Why: earlier versions of the IASR workbook contained the information needed to build these constraints; the v7.5 workbook does not. The full definitions (LHS coefficients, RHS values, etc.) still exist in AEMO's published PLEXOS model, so this script extracts them from the PLEXOS XML. It is a repeatable step that can be re-run when AEMO publishes a new model.

How: plexosdb loads the XML into an in-memory SQLite database, and a five-step query pipeline (constraint objects → memberships → data values → dates → tags) is merged into three long-format tables: constraints, lhs_terms, rhs_values. PLEXOS names, property names and the sense encoding are preserved verbatim; translation to ISPyPSA conventions is left to a follow-up templater PR. The module docstring documents the PLEXOS data model, which is otherwise difficult to review.

Validation: the PLEXOS XML is an undocumented relational-database dump, and the script's correctness depends on assumptions about its schema that cannot be fully verified from within the script. As a safeguard, before writing any output the script asserts a set of structural invariants against the real model — properties that must hold if those assumptions are correct (each constraint has exactly one Sense, an RHS and LHS terms; no bands; no unresolved references; and similar). These are not synthetic unit tests, which would only re-encode the assumptions being tested. A further cross-check against the simpler constraints still present in the IASR workbook is planned for the templater PR.

Includes a workaround for a plexosdb bug: AEMO's t_data.uid values overflow SQLite's 64-bit integers (reported upstream as issue #135).

Unit tests for the pure helpers are in tests/test_scripts/. The committed CSVs are the current 7.5 extract.

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[EllieKallmier]

yeah looks solid to me, I'm just getting into playing with plexosdb and less familiar with the plexos data structures and stuff so perhaps some of the detail is lost on me lol. But the tests and assertions throughout the script make lots of sense!

Testing my understanding here - this script doesn't get distributed with the package, but the CSV files output by the script will. So the script is like a dev tool to make a (probably) repeated process quicker, testable, and generally less prone to human error re: translating XML constraints to a CSV file (in this case).