Support ECDH with x25519 curve

[qmuntal]

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[Copilot]

Pull request overview

This PR adds support for ECDH (Elliptic Curve Diffie-Hellman) key exchange using the X25519 curve. X25519 is a Montgomery curve widely used for key agreement, and this change extends the existing ECDH implementation to support it alongside the existing NIST P-curves (P-256, P-384, P-521).

Key changes:

• Added X25519 constant definitions and OpenSSL bindings across multiple layers
• Extended ECDH key generation, import/export, and key agreement to handle X25519's raw key format
• Refactored ED25519 code to reuse common key extraction helpers

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
internal/ossl/zossl_nocgo.go	Added EVP_PKEY_Q_keygen_X25519 binding for no-CGO builds
internal/ossl/zossl_cgo.go	Added EVP_PKEY_Q_keygen_X25519 binding for CGO builds
internal/ossl/zossl.h	Added C function declaration and EVP_PKEY_X25519 constant
internal/ossl/zossl.go	Added EVP_PKEY_X25519 Go constant
internal/ossl/zossl.c	Added C wrapper implementation for X25519 key generation
internal/ossl/shims.h	Added X25519 constant and function signature with variadic attribute
evp.go	Integrated X25519 key generation into generateEVPPKey switch
ed25519.go	Refactored to use shared extractPKEYRawPublic helper
ecdh_test.go	Added comprehensive X25519 test coverage including vectors and edge cases
ecdh.go	Extended ECDH implementation to handle X25519's raw key format
ec.go	Added curveID function and extractPKEYRawPublic/extractPKEYRawPrivate helpers
const.go	Added _KeyTypeX25519 string constant

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