RFR: 8357913: Add `@Stable` to BigInteger and BigDecimal [v2]

[Shaojin Wen]

On Wed, 4 Jun 2025 14:03:49 GMT, Raffaello Giulietti <rgiulietti at openjdk.org> wrote:

>> Please do _not_ fix the `mag` issue in this PR.
>> I'll take care of it in a future PR.
>
> I just filed [JDK-8358599](https://bugs.openjdk.org/browse/JDK-8358599).

> Please do _not_ fix the `mag` issue in this PR. I'll take care of it in a future PR.

Note that there is a troublesome thing here. There are many places in BigInteger that pass mag to MutableBigInteger

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PR Review Comment: https://git.openjdk.org/jdk/pull/25437#discussion_r2126729841