Test parameter description punctuation, fix 178 violations (#340)

[vahid-ahmadi]

Summary

Closes #340 (and substantially addresses #339).

• New structural test policyengine_uk/tests/test_parameter_descriptions.py asserts every parameter `description` field ends with a period.
• Brings the existing 178 violating files into compliance with a one-shot mechanical edit (no semantic change).

Implementation notes

• Multi-line descriptions are handled correctly. YAML folded scalars like

  ```yaml
  description: Percentage of the Married Couple's Allowance which is deductible from
  Income Tax Liability
  ```

  parse to a single string. The fixer locates the last continuation line and appends `.` there, so the sentence ends correctly (`...Income Tax Liability.`) — not mid-clause.

• A handful of parameter files contain date-like keys that pyyaml's generic loader rejects with `year 0 is out of range`. The test catches `Exception` so pre-existing parse quirks don't block the assertion; PolicyEngine-core uses its own loader to read these at runtime.

Test plan

• `pytest policyengine_uk/tests/test_parameter_descriptions.py -v` — passes.
• `policyengine-core test policyengine_uk/tests/policy -c policyengine_uk` — 1000 passed (no regressions).
• Manual smoke: `CountryTaxBenefitSystem` builds, sample parameter descriptions render with trailing period (e.g. `Bank of England base rate.`, `Maximum weekly Industrial Injuries Disablement Benefit; amount varies in 10% increments.`).

🤖 Generated with Claude Code

[MaxGhenis]

No blocking findings. I checked the diff against current main and the PR merges cleanly with git merge-tree; the targeted structural test passed locally in a detached worktree:

uv run pytest policyengine_uk/tests/test_parameter_descriptions.py -q

Result: 1 passed, 1 warning.

Non-blocking note: the broad PyYAML exception path currently skips 12 known files with year-0/date parsing issues. If this test gets revisited later, making that skip list explicit would avoid silently excluding future parse failures from the punctuation assertion.