Add Unicode surrogate sanitization for all providers

Fixes issue where unpaired Unicode surrogates in tool results cause JSON serialization errors in API providers, particularly Anthropic.

- Add sanitizeSurrogates() utility function to remove unpaired surrogates
- Apply sanitization in all provider convertMessages() functions:
  - User message text content (string and text blocks)
  - Assistant message text and thinking blocks
  - Tool result output
  - System prompts
- Valid emoji (properly paired surrogates) are preserved
- Add comprehensive test suite covering all 8 providers

Previously only Google and Groq handled unpaired surrogates correctly.
Now all providers (Anthropic, OpenAI Completions/Responses, Google, xAI, Groq, Cerebras, zAI) sanitize text before API submission.

packages/ai/src/utils/sanitize-unicode.ts

@@ -0,0 +1,25 @@
+/**
+ * Removes unpaired Unicode surrogate characters from a string.
+ *
+ * Unpaired surrogates (high surrogates 0xD800-0xDBFF without matching low surrogates 0xDC00-0xDFFF,
+ * or vice versa) cause JSON serialization errors in many API providers.
+ *
+ * Valid emoji and other characters outside the Basic Multilingual Plane use properly paired
+ * surrogates and will NOT be affected by this function.
+ *
+ * @param text - The text to sanitize
+ * @returns The sanitized text with unpaired surrogates removed
+ *
+ * @example
+ * // Valid emoji (properly paired surrogates) are preserved
+ * sanitizeSurrogates("Hello 🙈 World") // => "Hello 🙈 World"
+ *
+ * // Unpaired high surrogate is removed
+ * const unpaired = String.fromCharCode(0xD83D); // high surrogate without low
+ * sanitizeSurrogates(`Text ${unpaired} here`) // => "Text  here"
+ */
+export function sanitizeSurrogates(text: string): string {
+	// Replace unpaired high surrogates (0xD800-0xDBFF not followed by low surrogate)
+	// Replace unpaired low surrogates (0xDC00-0xDFFF not preceded by high surrogate)
+	return text.replace(/[\uD800-\uDBFF](?![\uDC00-\uDFFF])|(?<![\uD800-\uDBFF])[\uDC00-\uDFFF]/g, "");
+}