Render escaped percent signs in TUI math (#57501)

## What changed

Recognize `\%` as a literal `%` in inline and display math, so expressions such as `$3.8\%$` render as `3.8%`.

## Testing

Extend inline and display math snapshots to cover escaped percent signs, including an inline expression with multiplication and a fraction.

GitOrigin-RevId: a0f9935dc8492fe1cd5a0d33d62930c48c5433b8

codex-rs/tui/src/markdown_render/math/render.rs

@@ -232,6 +232,7 @@ impl MathParser<'_> {
             return match self.take()? {
                 ',' | ';' | ':' | ' ' => Some(Layout::text(" ")),
                 '!' => Some(Layout::text("")),
+                '%' => Some(Layout::text("%")),
                 '{' => Some(Layout::text("{")),
                 '}' => Some(Layout::text("}")),
                 '|' => Some(Layout::text("‖")),

codex-rs/tui/src/markdown_render/math_display_tests.rs

@@ -29,6 +29,10 @@ $$
 
 $$\frac{1}{2}\frac{3}{4}$$
 
+$$
+3.8\%
+$$
+
 \[
 \int_0^1 x^2\,dx=\frac{1}{3}
 \]

codex-rs/tui/src/markdown_render/math_tests.rs

@@ -17,6 +17,7 @@ fn unicode_math_inline_snapshot() {
     insta::assert_snapshot!(plain(
         r"Inline $\alpha^2 + \beta_{10}$ on $\mathbb{R}^n$.
 Root $\sqrt{x^2+y^2}$ and fraction $a/\frac{b}{c}$.
+Percent: $3.8\%$ and $100000 \times 3.8\% \times \frac{360}{365} \approx 3747.95$.
 Unsupported: $\unknown{x_y}$ and $\frac{a}{b}^2$.
 Angles: <$x$>, <\(\alpha\)>, and <$\unknown{x}$>.
 Code: `$\alpha$`; money: $5 and $10; shell: $HOME.",

codex-rs/tui/src/markdown_render/snapshots/codex_tui__markdown_render__math__display_tests__unicode_math_snapshot.snap

@@ -12,6 +12,8 @@ x=────────────
 ─ ─
 2 4
 
+3.8%
+
 1        1
 ∫ x² dx= ─
 0        3

codex-rs/tui/src/markdown_render/snapshots/codex_tui__markdown_render__math__tests__unicode_math_inline_snapshot.snap

@@ -1,10 +1,11 @@
 ---
 source: tui/src/markdown_render/math_tests.rs
 assertion_line: 16
-expression: "plain(r\"Inline $\\alpha^2 + \\beta_{10}$ on $\\mathbb{R}^n$.\nRoot $\\sqrt{x^2+y^2}$ and fraction $a/\\frac{b}{c}$.\nUnsupported: $\\unknown{x_y}$ and $\\frac{a}{b}^2$.\nAngles: <$x$>, <\\(\\alpha\\)>, and <$\\unknown{x}$>.\nCode: `$\\alpha$`; money: $5 and $10; shell: $HOME.\",\n80)"
+expression: "plain(r\"Inline $\\alpha^2 + \\beta_{10}$ on $\\mathbb{R}^n$.\nRoot $\\sqrt{x^2+y^2}$ and fraction $a/\\frac{b}{c}$.\nPercent: $3.8\\%$ and $100000 \\times 3.8\\% \\times \\frac{360}{365} \\approx 3747.95$.\nUnsupported: $\\unknown{x_y}$ and $\\frac{a}{b}^2$.\nAngles: <$x$>, <\\(\\alpha\\)>, and <$\\unknown{x}$>.\nCode: `$\\alpha$`; money: $5 and $10; shell: $HOME.\",\n80)"
 ---
 Inline α² + β₁₀ on ℝⁿ.
 Root √(x²+y²) and fraction a/((b)/(c)).
+Percent: 3.8% and 100000 × 3.8% × ((360)/(365)) ≈ 3747.95.
 Unsupported: $\unknown{x_y}$ and $\frac{a}{b}^2$.
 Angles: <x>, <α>, and <$\unknown{x}$>.
 Code: $\alpha$; money: $5 and $10; shell: $HOME.