fix(render): restore trailing newlines stripped by markdown_to_ansi in stream

[np6126]

Summary

MarkdownStreamState::push calls markdown_to_ansi on each ready chunk, but render_markdown strips trailing newlines via trim_end() (see render.rs line 270). When the next streamed chunk arrives, its rendered output runs into the tail of the previous chunk in the terminal — adjacent paragraphs / list items / code blocks visibly merge into each other.

This PR restores the trailing newline(s) on the rendered output when the input chunk had them, matching the input's level of trailing whitespace (\n vs \n\n). flush() is unaffected since it returns the final chunk where trailing whitespace is irrelevant.

Diff

 pub fn push(&mut self, renderer: &TerminalRenderer, delta: &str) -> Option<String> {
     self.pending.push_str(delta);
     let split = find_stream_safe_boundary(&self.pending)?;
     let ready = self.pending[..split].to_string();
     self.pending.drain(..split);
-    Some(renderer.markdown_to_ansi(&ready))
+    let rendered = renderer.markdown_to_ansi(&ready);
+    // markdown_to_ansi strips trailing newlines via trim_end(); restore them
+    // so consecutive streamed chunks are separated in the terminal output.
+    let trailing = if ready.ends_with("\n\n") { "\n\n" } else { "\n" };
+    Some(if rendered.ends_with('\n') {
+        rendered
+    } else {
+        rendered + trailing
+    })
 }

Repro

Any OpenAI-compatible streaming endpoint whose deltas are separated by \n reproduces this — most providers do. Visual repro: stream a multi-line response with numbered lists or fenced code blocks and observe items concatenating without a visible separator.

Test plan

• cargo check --workspace clean
• Existing render tests: cargo test -p rusty-claude-cli
• Manual: stream a multi-line response through the CLI, observe newlines preserved between rendered chunks