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.

tools/transcription_tools.py

@@ -726,8 +726,8 @@ def _transcribe_xai(file_path: str, model_name: str) -> Dict[str, Any]:
     xai_config = stt_config.get("xai", {})
     base_url = str(
         xai_config.get("base_url")
-        or creds.get("base_url")
         or get_env_value("XAI_STT_BASE_URL")
+        or creds.get("base_url")
         or XAI_STT_BASE_URL
     ).strip().rstrip("/")
     language = str(