TabError and IndentationError from line continuation

Bug report

Bug description:

The documentation states:

Indentation cannot be split over multiple physical lines using backslashes; the whitespace up to the first backslash determines the indentation.

When this rule applies to some, but not all, lines in a block, a TabError is sometimes unexpectedly raised:

L1 = 'if 1:\n'
L2 = '    1\n'
L3 = '    \\\n'
L4 = '        1\n'
exec(L1 + L2 + L3 + L4)
# works as expected

L1 = 'if 1:\n'
L2 = '\t1\n'
L3 = '\t\\\n'
L4 = '\t\t1\n'
exec(L1 + L2 + L3 + L4)
# TabError: inconsistent use of tabs and spaces in indentation

Also, when a line containing only a backslash is explicitly joined with an indented line, an IndentationError is unexpectedly raised:

exec('\\\n    1')
# IndentationError: unexpected indent

CPython versions tested on:

3.10, 3.11, 3.12, 3.13, 3.14

Operating systems tested on:

macOS

先在列出的 CPython 版本上运行两个 exec() 复现程序，然后跟踪这两个案例中涉及的缩进和行继续处理。为报告的 TabError 和 IndentationError 行为添加回归测试覆盖，并确认测试捕获了预期结果，同时不改变无关的其他缩进错误。