Add test to ensure diversity in IsoFuelPopulation fallback

[shreyadesai20042009]

What this PR does

This PR adds a regression test for IsoFuelPopulation.generate
to cover the fallback behavior when the patcher returns fewer
routes than requested.

Why this is needed

Previously, when insufficient routes were generated, the fallback
logic cloned the last route repeatedly, resulting in an initial
population with identical individuals.

This test:

• Verifies the population size is preserved
• Ensures fallback routes are not identical clones
• Fails with the old cloning behavior
• Passes once diversity-preserving fallback logic is applied

Scope

• Tests only (no production code changes)
• Protects against future regressions

[shreyadesai20042009]

This PR adds tests only.
CI is awaiting maintainer approval to run on the fork.
Happy to make any adjustments if needed.

[kdemmich]

Hi @shreyadesai20042009, thank you for your PR. We appreciate the time and effort that was put into it but, unfortunately, the description does not follow our PR template. We will not evaluate PRs that do not follow our PR template.