refactor: consolidate _parse_retry_after_headers into base class

Merge the three retry-after header parsers (base, OpenAI, Anthropic)
into a single _extract_retry_after_from_headers on LLMProvider that
handles retry-after-ms, case-insensitive lookup, and HTTP date.

Remove the per-provider _parse_retry_after_headers duplicates and
their now-unused email.utils / time imports. Add test for retry-after-ms.

Made-with: Cursor

tests/providers/test_provider_retry.py

@@ -254,6 +254,14 @@ def test_extract_retry_after_from_headers_supports_numeric_and_http_date() -> No
     ) == 0.1
 
 
+def test_extract_retry_after_from_headers_supports_retry_after_ms() -> None:
+    assert LLMProvider._extract_retry_after_from_headers({"retry-after-ms": "250"}) == 0.25
+    assert LLMProvider._extract_retry_after_from_headers({"Retry-After-Ms": "1000"}) == 1.0
+    assert LLMProvider._extract_retry_after_from_headers(
+        {"retry-after-ms": "500", "retry-after": "10"},
+    ) == 0.5
+
+
 @pytest.mark.asyncio
 async def test_chat_with_retry_prefers_structured_retry_after_when_present(monkeypatch) -> None:
     provider = ScriptedProvider([