fix: combine session image and text into single Telegram message

When creating a session with an image, send it as a single SendPhoto
with the schedule text as caption (+ reply markup), instead of two
separate messages. Falls back to two messages if caption exceeds
Telegram's 1024-char limit.

Also adds BatchMessageEditor helper that transparently handles
EditMessageText vs EditMessageCaption depending on whether the batch
message is a text or photo message. Updated all handlers and web
service to use this helper.

Version bump to 1.9.7.

Directory.Build.props

@@ -1,6 +1,6 @@
 <Project>
   <PropertyGroup>
-    <Version>1.9.6</Version>
+    <Version>1.9.7</Version>
     <TargetFramework>net10.0</TargetFramework>
     <LangVersion>preview</LangVersion>
     <Nullable>enable</Nullable>