Convert non-BC ML-KEM keys in the legacy KEM code path

[WillChilds-Klein]

Written by Claude Opus 5 (Claude Code) on my behalf.

Description

• KemUtil looks up the ML-KEM key objects and the KeyGenerator from the JCA independently, so a provider ahead of BC can decode the peer's public key while BC still serves the encapsulation. MLKEMKeyGeneratorSpi casts to BCMLKEMPublicKey, so that split fails every handshake with internal_error(80) and no chained cause on the wire.
• The legacy encapsulate and decapsulate now re-import a foreign key through the KeyGenerator's own provider. decodePublicKey already does the equivalent when the KeyFactory is not BC's.
• Only the pre-JEP-452 path changes. Where SpiUtil.hasKEM() is true, javax.crypto.KEM accepts a foreign key already.
• The conversion is guarded on the generator belonging to a BouncyCastleProvider and the key not already implementing MLKEMPublicKey/MLKEMPrivateKey, so an all-BC classpath runs the same code as before.
• Both copies of KemUtil carry the fix, because the base copy is what runs when a repackaged jar leaves the multi-release overlay unreachable and SpiUtil.hasKEM() reports false on a KEM-capable JDK.

Testing / verification

• Drove two BCJSSE peers through a TLS 1.3 handshake on a KEM-capable JDK 17 with a third-party ML-KEM provider ahead of BC, over X25519MLKEM768, MLKEM768 and MLKEM1024. Each aborts with internal_error(80) before this change and completes after it.
• Covered both routes that reach the legacy path: a bcprov missing its versions/17 SpiUtil overlay next to a current bctls, and a fully flattened classpath where the base KemUtil runs.
• Re-ran the same legacy path with BC serving every ML-KEM primitive, to confirm the guard leaves the existing case untouched.
• The hybrid group exercises decapsulate as well, since the client generates its ephemeral keypair through the foreign provider and then extracts with BC.
• Happy to attach the standalone reproducer, or to fold it into tls tests behind a stub provider if you would like it in the suite.