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/conversation/llm/tokenization.rs

@@ -89,6 +89,7 @@ pub mod test {
 
         let message = super::Message {
             author: super::Author::User,
+            sender_id: None,
             content: super::MessageContent::Text("Hello there!".to_string()),
             timestamp: chrono::Utc::now(),
         };
@@ -108,6 +109,7 @@ pub mod test {
 
         let prompt = super::Message {
             author: super::Author::Prompt,
+            sender_id: None,
             content: super::MessageContent::Text("You are a bot!".to_string()),
             timestamp: chrono::Utc::now(),
         };
@@ -122,6 +124,7 @@ pub mod test {
 
         let first = super::Message {
             author: super::Author::User,
+            sender_id: None,
             content: super::MessageContent::Text("Hello there!".to_string()),
             timestamp: chrono::Utc::now(),
         };
@@ -136,6 +139,7 @@ pub mod test {
 
         let second = super::Message {
             author: super::Author::Assistant,
+            sender_id: None,
             content: super::MessageContent::Text("Hello!".to_string()),
             timestamp: chrono::Utc::now(),
         };
@@ -150,6 +154,7 @@ pub mod test {
 
         let third = super::Message {
             author: super::Author::User,
+            sender_id: None,
             content: super::MessageContent::Text(
                 "This is the 3rd message in this conversation. It shall be preserved.".to_owned(),
             ),
@@ -166,6 +171,7 @@ pub mod test {
 
         let forth = super::Message {
             author: super::Author::Assistant,
+            sender_id: None,
             content: super::MessageContent::Text(
                 "This is yet another message that shall be preserved.".to_owned(),
             ),
@@ -213,6 +219,7 @@ pub mod test {
 
         let prompt = super::Message {
             author: super::Author::User,
+            sender_id: None,
             content: super::MessageContent::Text("あなたはボットです。".to_string()),
             timestamp: chrono::Utc::now(),
         };
@@ -227,6 +234,7 @@ pub mod test {
 
         let first = super::Message {
             author: super::Author::User,
+            sender_id: None,
             content: super::MessageContent::Text("こんにちは!".to_string()),
             timestamp: chrono::Utc::now(),
         };
@@ -241,6 +249,7 @@ pub mod test {
 
         let second = super::Message {
             author: super::Author::Assistant,
+            sender_id: None,
             content: super::MessageContent::Text("こんにちは。今日は元気ですか。".to_string()),
             timestamp: chrono::Utc::now(),
         };
@@ -255,6 +264,7 @@ pub mod test {
 
         let third = super::Message {
             author: super::Author::User,
+            sender_id: None,
             content: super::MessageContent::Text(
                 "これは第3のメッセージなので、保存されます。".to_string(),
             ),
@@ -271,6 +281,7 @@ pub mod test {
 
         let forth = super::Message {
             author: super::Author::Assistant,
+            sender_id: None,
             content: super::MessageContent::Text(
                 "これはもう一つの保存されますメッセージです。".to_string(),
             ),