EN:

Extract isRateLimitedText, isContextExhaustedText, isVisionUnsupportedText,
sliceEqlAsciiFold, and containsAsciiFold into a neutral text_helpers.zig
module shared by both error_classify.zig and reliable.zig.

Changes:

• New: src/providers/text_helpers.zig (88 lines — neutral module, no provider imports)
• refactor: src/providers/error_classify.zig (445 → 357 lines, now imports reliable.zig)
• refactor: src/providers/reliable.zig (1555 → 1537 lines, delegates to text_helpers)

Why: Both files implemented the same text-matching predicates for error
classification. error_classify.zig used containsAsciiFold byte-by-byte; reliable.zig
used a single lowercase buffer. unified into one source of truth.

ZH:

将 isRateLimitedText、isContextExhaustedText、isVisionUnsupportedText、
sliceEqlAsciiFold 和 containsAsciiFold 提取到中立的 text_helpers.zig 模块，
由 error_classify.zig 和 reliable.zig 共同引用。

变更：

• 新增：src/providers/text_helpers.zig（88 行——中立模块，无 provider 导入）
• 重构：src/providers/error_classify.zig（445 → 357 行，现从 reliable.zig 导入）
• 重构：src/providers/reliable.zig（1555 → 1537 行，委托至 text_helpers）

**原因：**两个文件实现了相同的错误分类文本匹配断言。error_classify.zig
逐字节使用 containsAsciiFold；reliable.zig 使用单次小写转换缓冲。
统一为单一事实来源。

Validation:

zig build                    # clean
zig build test --summary all # 6098 passed, 11 skipped