fix: generate method assist uses enclosing impl block instead of first found

[akashchakrabortymsc-cmd]

Fixes #21550

Problem

When using the "Generate method" assist from inside an impl block,
the generated method was always appended to the first impl block
found for that type, instead of the impl block where the cursor is.

For example, given:

struct Foo;

impl Foo {
    fn new() -> Self { Foo }
}

impl Foo {
    fn method1(&self) {
        self.method2(42) // cursor here
    }
}

The generated method2 was incorrectly inserted into the first
impl Foo block instead of the one containing the cursor.

Fix

In gen_method, before falling back to get_adt_source, we now
check if the cursor is already inside an impl block for the target
type. If so, we use that impl block as the insertion target.

Test

Added a test case generate_method_uses_current_impl_block that
reproduces the issue and verifies the fix.

[akashchakrabortymsc-cmd]

Thank you for merging .Sorry about that in convenience .