replace tui_markdown with a custom markdown renderer (#3396)

Also, simplify the streaming behavior.

This fixes a number of display issues with streaming markdown, and paves
the way for better markdown features (e.g. customizable styles, syntax
highlighting, markdown-aware wrapping).

Not currently supported:
- footnotes
- tables
- reference-style links

codex-rs/tui/src/render/markdown_utils.rs

@@ -1,72 +0,0 @@
-/// Returns true if the provided text contains an unclosed fenced code block
-/// (opened by ``` or ~~~, closed by a matching fence on its own line).
-pub fn is_inside_unclosed_fence(s: &str) -> bool {
-    let mut open = false;
-    for line in s.lines() {
-        let t = line.trim_start();
-        if t.starts_with("```") || t.starts_with("~~~") {
-            if !open {
-                open = true;
-            } else {
-                // closing fence on same pattern toggles off
-                open = false;
-            }
-        }
-    }
-    open
-}
-
-/// Remove fenced code blocks that contain no content (whitespace-only) to avoid
-/// streaming empty code blocks like ```lang\n``` or ```\n```.
-pub fn strip_empty_fenced_code_blocks(s: &str) -> String {
-    // Only remove complete fenced blocks that contain no non-whitespace content.
-    // Leave all other content unchanged to avoid affecting partial streams.
-    let lines: Vec<&str> = s.lines().collect();
-    let mut out = String::with_capacity(s.len());
-    let mut i = 0usize;
-    while i < lines.len() {
-        let line = lines[i];
-        let trimmed_start = line.trim_start();
-        let fence_token = if trimmed_start.starts_with("```") {
-            "```"
-        } else if trimmed_start.starts_with("~~~") {
-            "~~~"
-        } else {
-            ""
-        };
-        if !fence_token.is_empty() {
-            // Find a matching closing fence on its own line.
-            let mut j = i + 1;
-            let mut has_content = false;
-            let mut found_close = false;
-            while j < lines.len() {
-                let l = lines[j];
-                if l.trim() == fence_token {
-                    found_close = true;
-                    break;
-                }
-                if !l.trim().is_empty() {
-                    has_content = true;
-                }
-                j += 1;
-            }
-            if found_close && !has_content {
-                // Drop i..=j and insert at most a single blank separator line.
-                if !out.ends_with('\n') {
-                    out.push('\n');
-                }
-                i = j + 1;
-                continue;
-            }
-            // Not an empty fenced block; emit as-is.
-            out.push_str(line);
-            out.push('\n');
-            i += 1;
-        } else {
-            out.push_str(line);
-            out.push('\n');
-            i += 1;
-        }
-    }
-    out
-}

codex-rs/tui/src/render/mod.rs

@@ -1,3 +1,2 @@
 pub mod highlight;
 pub mod line_utils;
-pub mod markdown_utils;