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/defaults.rs

@@ -1,5 +1,7 @@
 use super::{SpeechToTextFlowType, SpeechToTextMessageTypeForNonThreadedOnlyTranscribedMessages};
-use super::{TextGenerationAutoUsage, TextGenerationPrefixRequirementType};
+use super::{
+    TextGenerationAutoUsage, TextGenerationPrefixRequirementType, TextGenerationSenderContextMode,
+};
 use super::{TextToSpeechBotMessagesFlowType, TextToSpeechUserMessagesFlowType};
 
 pub const TEXT_GENERATION_PREFIX_REQUIREMENT_TYPE: TextGenerationPrefixRequirementType =
@@ -7,6 +9,9 @@ pub const TEXT_GENERATION_PREFIX_REQUIREMENT_TYPE: TextGenerationPrefixRequireme
 
 pub const TEXT_GENERATION_AUTO_USAGE: TextGenerationAutoUsage = TextGenerationAutoUsage::Always;
 
+pub const TEXT_GENERATION_SENDER_CONTEXT_MODE: TextGenerationSenderContextMode =
+    TextGenerationSenderContextMode::Disabled;
+
 pub const TEXT_TO_SPEECH_BOT_MESSAGES_FLOW_TYPE: TextToSpeechBotMessagesFlowType =
     TextToSpeechBotMessagesFlowType::OnDemandForVoice;