Misbehavior of `f.__code__.co_firstlineno` for decorated functions

Bug report

Bug description:

See also: #139783

Compare:

dummy = lambda f: f                 # L1
@dummy                              # L2
def f():                            # L3
    return                          # L4
print(f.__code__.co_firstlineno)    # L5  # result = 2

with

dummy = lambda f: f                 # L1
pass                                # L2
def f():                            # L3
    return                          # L4
print(f.__code__.co_firstlineno)    # L5  # result = 3

and

dummy = lambda f: f                 # L1
@\
dummy                               # L3
def f():                            # L4
    return                          # L5
print(f.__code__.co_firstlineno)    # L6  # result = 3

In the last example, the line continuation after @ is not handled correctly and the first line for f.__code__ then becomes the line at dummy. Because of that, the values returned by inspect.findsource(f) will not be consistent and inspect.getsourcelines will not use the correct source (it will only start at "dummy" and will not contain the "@" part).

@serhiy-storchaka suggested that this is an issue in the AST parser. I however don't know how we should actually fix this.

Consequences to inspect.getsource and other introspection functions:

import inspect

def outer(*args, **kwargs):
    def inner(f):
        return f
    return deco

@(
    outer(
        ...
    )
)
def f():
    return "hello"

print(inspect.getsource(f))

On Python 3.12, the output is expected:

@(
    outer(
        ...
    )
)
def func():
    return "hello"

On Python 3.13.7, the output is missing the first line @(:

    outer(
        ...
    )
)
def func():
    return "hello"

On Python 3.13.9 and main, we are missing @ and outer parentheses:

    outer(
        ...
    )

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

复现三个装饰器示例和嵌套装饰器示例，然后结合 inspect.findsource、inspect.getsourcelines 和 inspect.getsource 检查 AST 解析器的行为。完成的标准是：被装饰的函数报告正确的第一行源代码，包括跨行的装饰器和左括号，并且内省返回完整的装饰器源代码。