chore(deps): anthropic 1.5, openai 3.13, and the move to httpx2

Both SDKs' 1.x/3.x majors run on httpx2 (the maintained fork of httpx by its
original author, published by Pydantic at github.com/pydantic/httpx2, version
line 2.x) instead of httpx, which is what makes these two bumps one change.
httpx2 is now a declared dependency because two modules import it directly.
Everything else in the 0.x -> 1.x / 2.x -> 3.x change lists is absent here:
no Text Completions, no `with_raw_response`, no raw `output_format` dicts, no
Bedrock client, and `requires-python` is already 3.12.

Three code changes, all forced by the bump:

The BYOM DNS-pinning client in `docsgpt/security/safe_url.py` is handed to
`OpenAI(http_client=...)`, and the SDK rejects an old-httpx client at
construction — which would have taken the SSRF guard offline. It is built on
httpx2 now, and its `sni_hostname` extension carries a `str` rather than
ascii bytes: httpcore passes the value straight to
`ssl.SSLContext.wrap_socket`, and the truststore backend httpx2 uses for the
default system trust store encodes it instead of accepting bytes. Bytes
therefore failed every real handshake while passing the existing tests, which
stub the transport out; the test now pins the type and says why.

The stream-retry error tuple in `docsgpt/llm/base.py` named `httpx`
exceptions only. The two libraries' exception classes are unrelated types, so
after the bump the retry silently stopped firing for openai and anthropic
while still working for google-genai and elevenlabs. It now covers both
stacks, with a parametrized test over each.

anthropic 1.x dropped temperature/top_p/top_k from `messages.create`'s
signature (passing one raises TypeError) without dropping them from the API,
so the provider forwards them through `extra_body`. The wire request is
unchanged and a model that rejects them 400s exactly as before.

tests/llm/test_fallback.py

@@ -10,6 +10,7 @@ import types
 from unittest.mock import MagicMock
 
 import httpx
+import httpx2
 import pytest
 
 from docsgpt.llm.anthropic import AnthropicLLM
@@ -362,6 +363,36 @@ class TestStreamingFallback:
         assert primary.stream_calls == 2
         assert not backup.gen_stream_called
 
+    @pytest.mark.parametrize(
+        "error",
+        [httpx2.RemoteProtocolError, httpx.RemoteProtocolError],
+        ids=["httpx2", "httpx"],
+    )
+    def test_stream_transport_error_retries_on_either_http_stack(
+        self, patch_model_utils, error
+    ):
+        """The providers are split across two HTTP stacks whose exception
+        classes are unrelated types: openai and anthropic raise from httpx2,
+        google-genai and elevenlabs still from httpx. Naming one stack makes
+        the retry silently stop firing for the other half.
+        """
+        backup = FakeLLM(stream_chunks=["fallback"])
+        patch_model_utils(
+            get_provider=lambda m, **_kwargs: "openai",
+            get_api_key=lambda p: "k",
+            create_llm=lambda type, **kw: backup,
+        )
+        primary = FakeLLM(
+            stream_chunks=["ok1", "ok2"],
+            backup_models=["backup-model"],
+        )
+        primary.fail_schedule = [0, None]
+        primary.error_schedule = [error, RuntimeError]
+
+        assert list(primary.gen_stream(**CALL_ARGS)) == ["ok1", "ok2"]
+        assert primary.stream_calls == 2
+        assert not backup.gen_stream_called
+
     def test_stream_transport_error_retry_then_fails_uses_fallback(
         self, patch_model_utils
     ):