Add helper for validating finite, non-negative inputs

[alexdewar]

Description

There are loads of places in parameters.rs where we check that a number is finite and non-negative (I remembered this when I spotted another in #1463). Replace with a helper, deserialise_finite_non_negative, which works much like the existing deserialise_nonzero. This means we can remove duplicated code and some redundant tests, too.

Type of change

• Bug fix (non-breaking change to fix an issue)
• New feature (non-breaking change to add functionality)
• Refactoring (non-breaking, non-functional change to improve maintainability)
• Optimization (non-breaking change to speed up the code)
• Breaking change (whatever its nature)
• Documentation (improve or add documentation)

Key checklist

• All tests pass: $ cargo test
• The documentation builds and looks OK: $ cargo doc
• Update release notes for the latest release if this PR adds a new feature or fixes a bug
  present in the previous release

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 89.80%. Comparing base (2d4fd29) to head (0736385).

Files with missing lines	Patch %	Lines
src/input.rs	91.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1468   +/-   ##
=======================================
  Coverage   89.80%   89.80%           
=======================================
  Files          60       60           
  Lines        8443     8436    -7     
  Branches     8443     8436    -7     
=======================================
- Hits         7582     7576    -6     
  Misses        544      544           
+ Partials      317      316    -1     

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[Copilot]

Pull request overview

This PR refactors repeated “finite and non-negative” numeric validation into a shared Serde deserialisation helper, reducing duplicated validation code and associated tests across input and model-parameter parsing.

Changes:

• Added deserialise_finite_non_negative to centralise validation for finite, non-negative numeric inputs during deserialisation.
• Updated ModelParameters fields to use the new helper and removed now-redundant per-field validation functions/tests.
• Updated commodity demand CSV parsing to validate demand via the new deserialiser and removed redundant runtime checks/tests.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/model/parameters.rs	Uses the new deserialiser for several parameters and removes duplicated validation helpers/tests.
src/input/commodity/demand.rs	Moves demand non-negativity/finite validation into Serde deserialisation and drops redundant checks/tests.
src/input.rs	Introduces deserialise_finite_non_negative plus a focused unit test.

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