fix(markdown): 嵌套/未闭合代码块里的 \(...\) 被当成公式改写

src/components/ai-elements/math-delimiters.parse.test.ts

@@ -161,6 +161,32 @@ describe("remark-math with singleDollarTextMath: false", () => {
       expect(parseMath(normalizeMathDelimiters(text))).toEqual([])
     }
   })
+
+  it("does not rewrite \\(...\\) inside a fence nested in a longer fence", () => {
+    // ````md wraps a ```js block; the inner ``` must not close the outer
+    // ```` fence, so the code stays verbatim.
+    const text = "````md\n```js\nconst x = 1 // \\(x\\)\n```\n````"
+    expect(normalizeMathDelimiters(text)).toBe(text)
+    expect(parseMath(normalizeMathDelimiters(text))).toEqual([])
+  })
+
+  it("does not rewrite \\(...\\) inside a still-open fence (streaming mid-state)", () => {
+    // A code block whose closing fence has not streamed in yet runs to EOF;
+    // its contents must not be touched while it is transiently unclosed.
+    const text = "Here is the file:\n```tex\n\\(a+b\\)\n"
+    expect(normalizeMathDelimiters(text)).toBe(text)
+  })
+
+  it("still rewrites \\(...\\) in prose between two fenced blocks", () => {
+    const text =
+      "```\ncode1 \\(keep\\)\n```\nprose \\(x\\) here\n```\ncode2\n```"
+    const out = normalizeMathDelimiters(text)
+    expect(out).toContain("$$x$$")
+    expect(out).toContain("\\(keep\\)")
+    expect(parseMath(out)).toEqual([
+      { type: "inlineMath", value: "x", meta: null },
+    ])
+  })
 })
 
 describe("block structure around a normalized formula", () => {

src/components/ai-elements/message.tsx

@@ -381,8 +381,16 @@ export function normalizeMathDelimiters(text: string): string {
     saved.push(m)
     return `\0CBLK${saved.length - 1}\0`
   }
+  // A fenced block closes only on a fence of the SAME char run at least as
+  // long as its opener (CommonMark 4.5), and an unclosed fence runs to EOF.
+  // Pairing any 3+ run with the next one (the old `` `{3,}[\s\S]*?`{3,} ``)
+  // mis-split a ```` ````md ```` block at its inner ``` ``` ```` fence and left
+  // the code inside unmasked, so `\(...\)` there got rewritten — and every
+  // streaming code block hit the same rewrite while its closing fence was
+  // still pending. Capture the opener and require the closer to be `\1`+ (or
+  // end of string).
   const masked = text.replace(
-    /`{3,}[\s\S]*?`{3,}|~{3,}[\s\S]*?~{3,}|`[^`\n]+`/g,
+    /(`{3,})[\s\S]*?(?:\1`*|$)|(~{3,})[\s\S]*?(?:\2~*|$)|`[^`\n]+`/g,
     placeholder
   )
   // Fold line endings only after masking, so the offsets and line scans below