feat(codechef): implement full CodeChef support (#354)

## Problem

CodeChef had no working login, submit, or contest list. The browser
selectors were wrong, the contest list was missing present/past
contests,
and problem/contest URLs were unset.

## Solution

Fix login and submit selectors for the Drupal-based site. Paginate
`/api/list/contests/past` to collect all 228 Starters, then expand each
parent contest into individual division entries (e.g. `START228 (Div.
4)`).
Add language IDs, correct `url`/`contest_url`/`standings_url` in
metadata,
and make `:CP <platform>` open the contest picker directly.

tests/conftest.py

@@ -221,6 +221,9 @@ def run_scraper_offline(fixture_text):
                     if "/api/list/contests/all" in url:
                         data = json.loads(fixture_text("codechef/contests.json"))
                         return MockResponse(data)
+                    if "/api/list/contests/past" in url:
+                        data = json.loads(fixture_text("codechef/contests_past.json"))
+                        return MockResponse(data)
                     if "/api/contests/START" in url and "/problems/" not in url:
                         contest_id = url.rstrip("/").split("/")[-1]
                         try:

tests/fixtures/codechef/contests_past.json

@@ -0,0 +1,16 @@
+{
+  "status": "success",
+  "message": "past contests list",
+  "contests": [
+    {
+      "contest_code": "START209D",
+      "contest_name": "Starters 209 Div 4",
+      "contest_start_date_iso": "2025-01-01T10:30:00+05:30"
+    },
+    {
+      "contest_code": "START208",
+      "contest_name": "Starters 208",
+      "contest_start_date_iso": "2024-12-25T10:30:00+05:30"
+    }
+  ]
+}