fix: detect a selected-but-unusable OS keyring and fall back gracefully (#10168)

evaluate_and_patch_config() only disabled USE_OS_SECRET_STORAGE when
keyring.get_keyring().name == 'fail Keyring'. On Debian 13 over RDP, a
real backend (e.g. SecretService) is selected because no D-Bus/GNOME
Keyring session is running, so the name check passes even though every
actual keyring call fails — leaving the crypt key unset and every
operation raising a bare CryptKeyMissing.

Probe the selected backend with a harmless read of a never-existing
entry after selection. A healthy backend returns None without
prompting; an unusable one raises, and we disable
USE_OS_SECRET_STORAGE so the app falls back to the master-password /
in-app crypt key mechanism instead of failing outright.

Fixes #10107

web/pgadmin/evaluate_config.py

@@ -138,8 +138,26 @@ def evaluate_and_patch_config(config: dict) -> dict:
         config.setdefault('KEYRING_NAME', '')
     else:
         k_name = keyring.get_keyring().name
-        # Setup USE_OS_SECRET_STORAGE false as no keyring backend available
-        if k_name == 'fail Keyring':
+        # A keyring backend may be selected (e.g. SecretService on Linux)
+        # yet be completely unusable at runtime - for example in a headless
+        # or RDP session where there is no running D-Bus / GNOME Keyring.
+        # In that case every keyring call raises and the crypt key is never
+        # set, surfacing as a bare CryptKeyMissing on every operation.
+        # Probe the backend with a harmless read of an entry that never
+        # exists; if the backend is healthy this returns None without
+        # prompting, and if it is unusable it raises. When it is not usable,
+        # disable OS secret storage so pgAdmin falls back to the
+        # master-password / in-app crypt key mechanism.
+        keyring_usable = k_name != 'fail Keyring'
+        if keyring_usable:
+            try:
+                keyring.get_password(
+                    'pgAdmin4', 'entry_to_check_keyring_access')
+            except Exception:
+                keyring_usable = False
+
+        if not keyring_usable:
+            # Setup USE_OS_SECRET_STORAGE false as no usable keyring backend
             config['USE_OS_SECRET_STORAGE'] = False
             config['KEYRING_NAME'] = ''
         else: