kconfig: fix extra output from savedefconfig on out-of-range defaults

The Kconfig interpreter currently allows defaults that are outside of the
range bounds.

In these cases, the 'sym_validate_range' function will adjust the default
value to the nearest range bound. For example, see this example:

  config A
    int
    range 1 2
    default 16

Here, since the default value of 16 is greater than the bounds, the
effective default value gets adjusted down to the upper bound, 2.

However, 'savedefconfig' writes non-default values, and without being
aware of the automatic adjustment to the range bound, it would write: A=2

This limitation is also documented in a comment: "The following fails to
handle the situation where a default value is further limited by the valid
range."

To resolve this, let's factor out the default-range adjustment logic from
the existing 'sym_validate_range' function into its own
'sym_get_near_range_bound' function for 'savedefconfig' to use too, so
that it compares against the effective value.

Adds tests, accordingly.

Fixes: 7cf3d73b4360 ("kconfig: add savedefconfig")
Assisted-by: Codex:gpt-5.6-sol
Reported-by: Geert Uytterhoeven <geert+renesas@glider.be>
Closes: https://lore.kernel.org/lkml/CAMuHMdVyUAA3L4mUkSjmnuE3cvj-+N8z-Bhxsh1wa-FQWc=fjw@mail.gmail.com/
Signed-off-by: Julian Braha <julianbraha@gmail.com>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Link: https://patch.msgid.link/20260830221129.2668354-1-julianbraha@gmail.com
Signed-off-by: Nathan Chancellor <nathan@kernel.org>

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