Add sender context mode for text generation (#104)

Add a per-room/global `sender_context_mode` setting that optionally
prefixes conversation messages with sender metadata before sending them
to the model provider.

This helps models distinguish between participants in multi-user rooms.
                                                                                                                                                                                                                                                             
Three modes are supported:
- `disabled` (default, no change)
- `matrix_user_id` (prefixes with `[sender=@user:server]`)
- `matrix_user_id_and_timestamp` (adds `send_at`; Example: `[sender=@user:server sent_at=<ISO 8601>]`)
                                                                                                                                                                                                                                                             
Sender context is applied to user and assistant text messages only,
skipping system prompts and non-text content.

Mixed-sender merged turns (something we intentionally do for Anthropic)
have their `sender_id` cleared to avoid misattribution.

src/entity/roomconfig/mod.rs

@@ -6,8 +6,8 @@ use mxlink::helpers::account_data_config::RoomConfigManager as AccountDataRoomCo
 pub use entity::{RoomConfig, RoomConfigCarrierContent, RoomSettings, RoomSettingsHandler};
 pub use entity::{
     SpeechToTextFlowType, SpeechToTextMessageTypeForNonThreadedOnlyTranscribedMessages,
-    TextGenerationAutoUsage, TextGenerationPrefixRequirementType, TextToSpeechBotMessagesFlowType,
-    TextToSpeechUserMessagesFlowType,
+    TextGenerationAutoUsage, TextGenerationPrefixRequirementType, TextGenerationSenderContextMode,
+    TextToSpeechBotMessagesFlowType, TextToSpeechUserMessagesFlowType,
 };
 
 pub type RoomConfigurationManager =