fix: escape regex keys in substituteVariables and await Hive writes in history providers

[nazifsco]

Fixes two of the three issues reported in #1642.

Fix 1 -- envvar_utils.dart

substituteVariables built a regex from env var keys without escaping them. Keys containing regex metacharacters like api.key (dot) or base+url (plus) caused a FormatException crash before the request was sent.

// Before
final regex = RegExp("{{(${envVarMap.keys.join('|')})}}");

// After
final escapedKeys = envVarMap.keys.map(RegExp.escape).join('|');
final regex = RegExp("{{($escapedKeys)}}");

Fix 2 -- history_providers.dart

addHistoryRequest and editHistoryRequest called setHistoryIds() and setHistoryMeta() without await. This created a race condition where the app could be closed before the history index was persisted, leaving orphaned records and causing data loss on restart. Also changed both method return types from void to Future<void>.

// Before
hiveHandler.setHistoryIds(updatedHistoryKeys);
hiveHandler.setHistoryMeta(id, model.metaData.toJson());

// After
await hiveHandler.setHistoryIds(updatedHistoryKeys);
await hiveHandler.setHistoryMeta(id, model.metaData.toJson());

Note: the third issue in #1642 (getEnabledRows duplicate row bug) was already fixed in #1465.