fix(python): PLR0915 counts statements, not lines

Verified with `ruff rule PLR0915`: it is `too-many-statements` (default 50),
not a line-length rule. Ruff has no line-based function-length rule at all.
python.md now says so and points both limits at a pytest case; coding-dev.md
gains the general distinction between a real length lint and a proxy.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

coding-dev.md

@@ -147,8 +147,10 @@ depends on those.
 A limit nobody checks is a suggestion. Pick the **highest** applicable option:
 
 1. **A linter rule**, where one exists — the function limit is usually covered natively
-   (Clippy's `too_many_lines`, ESLint's `max-lines-per-function`, `golangci-lint`'s `funlen`,
-   Ruff's `PLR0915`). A function that trips it wants extracting, not a suppression.
+   (Clippy's `too_many_lines`, ESLint's `max-lines-per-function`, `golangci-lint`'s `funlen`).
+   A function that trips it wants extracting, not a suppression. Note that some linters only
+   offer *proxies* for length — Ruff's `PLR0915` counts statements, not lines — in which case
+   enable the proxy and put the real limit in a test.
 2. **A test**, for anything the linter does not cover — file length almost always falls here.
    A test is the best home for it: it runs with the suite everyone already runs, it needs no
    extra tooling, it fails with a message naming the offending files, and it travels with the