Implement more fine-grained typing notices sending

The previous approach (implemented in dd1dd78312) was simple
(send typing notices for as long as the "controller" is running),
but this proved to be overly simplistic and unable to handle edge-cases:

- in multi-user rooms (or rooms with a prefix requirement), the bot
  used to send a typing notice while "working", but its work consisted
  of ignoring the message. So it then sent a "not typing" notice.
  This is wasteful and otherwise problematic - certain clients (like nheko)
  do not handle this "race" well.

- certain reactions (anything other than 🗣️ right now) are meant to be
  ignored. There's no point in doing the same "typing / not typing"
  dance

- there are other instances where the bot may do work, but doesn't (due
  to configuration or lack of capabilities)

This new more fine-grained implementation of typing notices aims to:

- only send a typing notice if actual "slow work" will be done

- avoid stopping & restarting typing notices (wasteful) if a chain of work is to
  be performed (processing voice messages and doing speech-to-text +
  text-generation + ...). Rather, maintaining typing notice sending
  throughout

src/controller/chat_completion/mod.rs

@@ -43,6 +43,8 @@ pub async fn handle(
 ) -> anyhow::Result<()> {
     let mut original_message_is_audio = false;
 
+    let mut _typing_notice_guard: Option<mxlink::TypingNoticeGuard> = None;
+
     let speech_to_text_flow_type = message_context
         .room_config_context()
         .speech_to_text_flow_type();
@@ -71,6 +73,10 @@ pub async fn handle(
             }
         };
 
+        if _typing_notice_guard.is_none() {
+            _typing_notice_guard = Some(bot.start_typing_notice(message_context.room()).await);
+        }
+
         let Some(speech_to_text_created_event_id_result) =
             handle_stage_speech_to_text(bot, message_context, audio_content, response_type).await
         else {
@@ -96,6 +102,10 @@ pub async fn handle(
         .room_config_context()
         .should_auto_text_generate(original_message_is_audio)
     {
+        if _typing_notice_guard.is_none() {
+            _typing_notice_guard = Some(bot.start_typing_notice(message_context.room()).await);
+        }
+
         let speech_to_text_created_event_id_reaction_event_id =
             if let Some(speech_to_text_created_event_id) = speech_to_text_created_event_id {
                 let reaction_event_response = bot
@@ -213,6 +223,10 @@ pub async fn handle(
 
     match text_to_speech_stage_params {
         Some(TextToSpeechParams::Perform(text_to_speech_eligible_payload, response_type)) => {
+            if _typing_notice_guard.is_none() {
+                _typing_notice_guard = Some(bot.start_typing_notice(message_context.room()).await);
+            }
+
             let _tts_result = generate_and_send_tts_for_message(
                 bot,
                 matrix_link.clone(),