fix: strip all trailing assistant messages without tool_calls

Strengthens the strip to remove any trailing assistant message (not
just empty ones) when it has no tool_calls. The Copilot proxy for
Claude rejects conversations ending with any assistant message as
unsupported prefill. This fixes the Telegram bot channel where the
agent loop appends an assistant message with content.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

crates/openfang-runtime/src/drivers/openai.rs

@@ -262,18 +262,13 @@ struct OaiUsage {
     completion_tokens: u64,
 }
 
-/// Strip trailing empty assistant messages from the message list.
-/// Some API proxies (e.g. Copilot proxying Claude/Gemini) reject these as
-/// unsupported "assistant message prefill".
+/// Strip trailing assistant messages without tool calls from the message list.
+/// The Copilot API proxy for Claude rejects conversations ending with an
+/// assistant message as unsupported "assistant message prefill". Assistant
+/// messages with tool_calls are kept (they're part of the tool call protocol).
 fn strip_trailing_empty_assistant(messages: &mut Vec<OaiMessage>) {
     while messages.last().map_or(false, |m| {
-        m.role == "assistant"
-            && m.tool_calls.is_none()
-            && match &m.content {
-                None => true,
-                Some(OaiMessageContent::Text(t)) => t.trim().is_empty(),
-                _ => false,
-            }
+        m.role == "assistant" && m.tool_calls.is_none()
     }) {
         messages.pop();
     }