fix(config_validate): add missing aliases/providerFallbacks to schema; fix deprecated-key bypass

Two real schema gaps found via dogfood (cargo test -p runtime):

1. aliases and providerFallbacks not in TOP_LEVEL_FIELDS
   - Both are valid config keys parsed by config.rs
   - Validator was rejecting them as unknown keys
   - 2 tests failing: parses_user_defined_model_aliases,
     parses_provider_fallbacks_chain

2. Deprecated keys were being flagged as unknown before the deprecated
   check ran (unknown-key check runs first in validate_object_keys)
   - Added early-exit for deprecated keys in unknown-key loop
   - Keeps deprecated→warning behavior for permissionMode/enabledPlugins
     which still appear in valid legacy configs

3. Config integration tests had assertions on format strings that never
   matched the actual validator output (path:3: vs path: ... (line N))
   - Updated assertions to check for path + line + field name as
     independent substrings instead of a format that was never produced

426 tests passing, 0 failing.

rust/crates/runtime/src/config_validate.rs

@@ -185,6 +185,14 @@ const TOP_LEVEL_FIELDS: &[FieldSpec] = &[
         name: "env",
         expected: FieldType::Object,
     },
+    FieldSpec {
+        name: "aliases",
+        expected: FieldType::Object,
+    },
+    FieldSpec {
+        name: "providerFallbacks",
+        expected: FieldType::Object,
+    },
 ];
 
 const HOOKS_FIELDS: &[FieldSpec] = &[
@@ -364,6 +372,8 @@ fn validate_object_keys(
                     },
                 });
             }
+        } else if DEPRECATED_FIELDS.iter().any(|d| d.name == key) {
+            // Deprecated key — handled separately, not an unknown-key error.
         } else {
             // Unknown key.
             let suggestion = suggest_field(key, &known_names);