refactor(hermes): simplify schema handling + preserve unknown provider fields

Drops the v11→v12 providers-dict compat layer: CC Switch now only
reads/writes `custom_providers:`, leaving migrated `providers:` dict
entries to Hermes Web UI for reconciliation (Hermes' runtime already
merges both shapes via `get_compatible_custom_providers`). The
`schema_migrated_v12` health warning now points users there when a
dict-migrated config is detected.

Adds forward-compat merge to `set_provider`: when updating an existing
entry, on-disk fields the UI payload didn't submit (e.g. Hermes-only
`request_timeout_seconds`, `key_env`) are carried over. Without this,
editing one field via CC Switch would silently strip the rest.

Adds `set_memory_enabled` + `set_hermes_memory_enabled` Tauri command
for the upcoming memory-switch UI. Writes go through a merge-aware
section replacement so character budgets and external-provider fields
survive toggle operations.

Removes four dict-only helpers (`normalize_providers_dict_entry_for_read`,
`rename_alias_key`, `json_obj_non_empty_str`,
`resolve_provider_name_from_yaml_entry`) and the multi-section write
helper. Simplifies `get_providers` / `remove_provider` / health scan
back to list-only. Replaces nine obsolete dict-related tests with
`set_provider_preserves_unknown_fields_on_update` and
`set_memory_enabled_preserves_other_fields`.

src-tauri/src/lib.rs

@@ -1261,6 +1261,7 @@ pub fn run() {
             commands::get_hermes_memory,
             commands::set_hermes_memory,
             commands::get_hermes_memory_limits,
+            commands::set_hermes_memory_enabled,
             // Global upstream proxy
             commands::get_global_proxy_url,
             commands::set_global_proxy_url,