Match host, not substring, when resolving DuckDuckGo redirects (#886)

_resolve_ddg_redirect (the DuckDuckGo /l/?uddg= redirect resolver used on every
HTML-fallback result href) gated on `"duckduckgo.com" in parsed.hostname`. That
substring test also matches look-alike hosts like `duckduckgo.com.evil.com` and
`notduckduckgo.com`, so a result link on such a host would be silently rewritten
to its embedded `uddg` target. Same substring-vs-hostname pitfall fixed for
provider detection in 54ecfa3.

Match the host properly: exactly `duckduckgo.com` or a `.duckduckgo.com`
subdomain. Genuine redirects (`//duckduckgo.com/l/...`, and relative `/l/...`
hrefs resolved against `html.duckduckgo.com`) keep working.

The resolver was a closure inside duckduckgo_search; lifted it (plus the new
_is_duckduckgo_host helper) to module scope so it can be unit-tested directly.

Adds tests/test_ddg_redirect_resolution.py (red on the look-alike case before
this change, green after).

tests/test_ddg_redirect_resolution.py

@@ -0,0 +1,37 @@
+"""Resolving DuckDuckGo /l/?uddg= redirects must match the host, not a substring.
+
+`_resolve_ddg_redirect` only extracts the embedded `uddg` destination when the
+redirect link is actually on DuckDuckGo. The host check used
+`"duckduckgo.com" in parsed.hostname`, which also matches look-alike hosts such
+as `duckduckgo.com.evil.com` or `notduckduckgo.com` — so a result link on one of
+those would be silently rewritten to its embedded `uddg` target. Same
+substring-vs-hostname pitfall fixed for provider detection in 54ecfa3.
+"""
+from src.search.providers import _resolve_ddg_redirect, _is_duckduckgo_host
+
+
+def test_resolves_genuine_ddg_redirects():
+    # protocol-relative DDG redirect
+    assert _resolve_ddg_redirect(
+        "//duckduckgo.com/l/?uddg=https%3A%2F%2Fexample.com"
+    ) == "https://example.com"
+    # relative href -> resolved against html.duckduckgo.com (a real DDG subdomain)
+    assert _resolve_ddg_redirect(
+        "/l/?uddg=https%3A%2F%2Fexample.com"
+    ) == "https://example.com"
+
+
+def test_ignores_lookalike_hosts():
+    for host in ("duckduckgo.com.evil.com", "notduckduckgo.com"):
+        url = f"https://{host}/l/?uddg=https%3A%2F%2Fexample.com"
+        # Must be returned unchanged — it is NOT a DuckDuckGo redirect.
+        assert _resolve_ddg_redirect(url) == url
+
+
+def test_host_matcher():
+    assert _is_duckduckgo_host("duckduckgo.com")
+    assert _is_duckduckgo_host("html.duckduckgo.com")
+    assert _is_duckduckgo_host("lite.duckduckgo.com")
+    assert not _is_duckduckgo_host("duckduckgo.com.evil.com")
+    assert not _is_duckduckgo_host("notduckduckgo.com")
+    assert not _is_duckduckgo_host("")