fix(web): don't crash on a null web/backend config value

`_load_web_config()` is typed `-> dict` but returned `load_config().get("web",
{})`, which is `None` when the config has a present-but-null `web:` section
(YAML `web:` with no body). Every caller then does
`_load_web_config().get(...)` and raises `AttributeError: 'NoneType' object
has no attribute 'get'` — this hits `_get_backend`, `check_web_api_key`, and
the extract-char-limit reader.

Separately, `check_web_api_key()` read the backend as
`.get("backend", "").lower()`; a null `web.backend` value yields `None` (the
`""` default only applies when the key is absent), so `None.lower()` raised.
`check_web_api_key` is the `check_fn` gate for `web_search`/`web_extract`, so
this surfaced as an exception during tool-availability checking.

- Make `_load_web_config()` honor its `-> dict` contract (`... or {}`), fixing
  the null-`web:`-section crash at every call site.
- Guard the backend value in `check_web_api_key` with `or ""`, mirroring the
  existing guard in `_get_backend`.

Adds regression tests for both the null-backend-value and null-web-section
cases.

tools/web_tools.py

@@ -132,7 +132,11 @@ def _load_web_config() -> dict:
     """Load the ``web:`` section from ~/.hermes/config.yaml."""
     try:
         from hermes_cli.config import load_config
-        return load_config().get("web", {})
+        # ``or {}``: a present-but-null ``web:`` section (YAML ``web:`` with no
+        # body) makes ``.get("web", {})`` return None, which would break every
+        # caller that does ``_load_web_config().get(...)``. Honor the ``-> dict``
+        # contract so callers never see None.
+        return load_config().get("web") or {}
     except (ImportError, Exception):
         return {}
 
@@ -1004,7 +1008,9 @@ def check_web_api_key() -> bool:
     :func:`_is_backend_available`, which delegates non-legacy names to the
     registry.
     """
-    configured = _load_web_config().get("backend", "").lower().strip()
+    # ``or ""``: a null ``web.backend`` value yields None from ``.get``, and
+    # ``None.lower()`` would raise. Mirrors ``_get_backend``.
+    configured = (_load_web_config().get("backend") or "").lower().strip()
     if configured and _is_backend_available(configured):
         return True
     # Any built-in backend with credentials present. This is a boolean OR, so