Take the ITS explicit certificate signature choice from the content signer's curve

[Arpan0995]

ITSExplicitCertificateBuilder.build picks the certificate's Signature CHOICE by switching a VerificationKeyIndicator CHOICE, the certificate's own when self-signing and otherwise the issuer's, against the PublicVerificationKey curve constants (ITSExplicitCertificateBuilder.java:65-75, :91-104). The two CHOICE spaces are unrelated: verificationKey = 0 and reconstructionValue = 1 (VerificationKeyIndicator.java:25-26) index ecdsaNistP256 and ecdsaBrainpoolP256r1 (PublicVerificationKey.java:24-26), so the signing key's curve never enters the decision:

• a self-signed brainpoolP256r1 certificate is tagged ecdsaNistP256Signature, and the verifier picked by that tag (ITSCertificate.java:57) refuses it with OperatorCreationException: wrong verifier for algorithm: 0 (BcITSContentVerifierProvider.java:83-86);
• no certificate can be signed with a brainpoolP384r1 key: the NIST arm applies the 32-byte encoding at ECDSAEncoder.java:60 to a 384-bit r, and 25 of 25 self-signed attempts threw IllegalArgumentException: standard length exceeded for value. A brainpoolP384r1 subject key under a P-256 issuer is unaffected;
• an explicit certificate under an implicit issuer takes tag 1 from reconstructionValue = 1: a secp256r1 signature is silently mislabelled ecdsaBrainpoolP256r1Signature, a brainpoolP256r1 one happens to be right, and a brainpoolP384r1 signer throws as above.

Reproduced on the released bcpkix-jdk18on-1.86.jar, on current origin/main (94270ff1cb) and on the byte-identical 1.87-SNAPSHOT beta (1.87.0.20719), with the same output and the re-decoded OER carrying the same tag. build takes no curve argument, so there is no workaround; every path is fail-closed. ETSISignedDataBuilder reads the curve from the signer already (ETSISignedDataBuilder.java:104, :131, :153).

This change:

• takes the curve from the content signer, ECDSAEncoder.toITS(signer.getCurveID(), signer.getSignature()), dropping two locals and three imports. Both shipped signers reject a fourth curve at construction with IllegalArgumentException: unknown key type (BcITSContentSigner.java:64, JcaITSContentSigner.java:101); a third-party ITSContentSigner reporting one now gets IllegalArgumentException: unknown curveID (ECDSAEncoder.java:76) rather than the removed arm's IllegalStateException. JcaITSExplicitCertificateBuilder inherits the corrected build; and
• adds testBuildSelfSignedCurves and testBuildUnderImplicitIssuer to ITSBasicTest: the first builds a self-signed CA and an end-entity certificate under it on all three curves, checking the emitted CHOICE and that the verifier accepts each; the second pins the implicit-issuer case. Both fail without the change (ITSBasicTest.java:366, :452) and pass with it; the package's 21 pre-existing tests pass either way.

Base tree only, no overlays and no module-info change (org.bouncycastle.its is already exported). A release-note entry is included.