fix(xai-http): preserve ~/.hermes/.env fallback and XAI_STT_BASE_URL precedence

The new resolve_xai_http_credentials() resolver was using os.getenv()
for the XAI_API_KEY/XAI_BASE_URL fallback path, which dropped the
~/.hermes/.env contract guarded by PR #17140 / #17163. Users with
XAI_API_KEY in dotenv only would see "No xAI credentials found" even
though the key was configured.

Separately, _transcribe_xai started consulting creds["base_url"] (which
always returns at least the default https://api.x.ai/v1) ahead of the
public XAI_STT_BASE_URL env override, so the per-tool override stopped
working.

- tools/xai_http.py: add module-level get_env_value() wrapper that
  reads ~/.hermes/.env first (via hermes_cli.config.get_env_value),
  then os.environ. Resolver uses it for the API-key/base-url fallback.
- tools/transcription_tools.py: restore precedence so XAI_STT_BASE_URL
  wins over creds["base_url"].
- tests/tools/test_transcription_dotenv_fallback.py +
  tests/tools/test_tts_dotenv_fallback.py: repoint the per-call-site
  patches at the new resolution point (tools.xai_http.get_env_value).
  The end-to-end regression-guard test (which patches load_env) is
  unchanged and still passes.

tests/tools/test_tts_dotenv_fallback.py

@@ -57,7 +57,12 @@ class TestDotenvFallbackPerProvider:
             mock_import.return_value.assert_called_once_with(api_key="el-dotenv-key")
 
     def test_xai_reads_dotenv_key(self, tmp_path):
+        """xAI TTS now resolves credentials through ``tools.xai_http``; the
+        dotenv fallback contract from #17140 is preserved by patching the
+        resolver's ``get_env_value`` rather than ``tts_tool.get_env_value``.
+        """
         from tools import tts_tool
+        from tools import xai_http
 
         captured: dict = {}
 
@@ -69,7 +74,7 @@ class TestDotenvFallbackPerProvider:
             response.raise_for_status = MagicMock()
             return response
 
-        with patch.object(tts_tool, "get_env_value", return_value="xai-dotenv-key"), \
+        with patch.object(xai_http, "get_env_value", return_value="xai-dotenv-key"), \
              patch("requests.post", side_effect=fake_post):
             tts_tool._generate_xai_tts("hi", str(tmp_path / "out.mp3"), {})