`csv.Sniffer.sniff()` raises `Could not determine delimiter` when the sample contains a field larger than field_size_limit()

Bug report

Bug description:

Summary

csv.Sniffer().sniff() raises csv.Error: Could not determine delimiter for a sample whose first row contains a field longer than the current csv.field_size_limit() (131072 by default), even when every following row is consistently delimited.

Reproduction Code

import csv 
 
sample = 'a,"' + 'x' * 200_000 + '",b\n' + 'c,d,e\n' * 50 
print(csv.Sniffer().sniff(sample).delimiter) 

Actual Behavior

Traceback (most recent call last): 
  File "repro.py", line 4, in <module> 
    print(csv.Sniffer().sniff(sample).delimiter) 
          ~~~~~~~~~~~~~~~~~~~^^^^^^^^ 
  File "Lib/csv.py", line 384, in sniff 
    raise Error("Could not determine delimiter") 
_csv.Error: Could not determine delimiter 

Expected Behavior

sniff() should return a dialect with `delimiter=','

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-157689

使用 issue 中的示例重现该失败，然后检查 Lib/csv.py 中 Sniffer.sniff() 附近的代码。检查 field_size_limit 如何影响分隔符检测，并验证该示例返回的 dialect 的分隔符是逗号，同时不削弱限制行为。issue 中链接的 PR gh-157689 表明这项工作可能已经在进行中。