Add kex integration test for ed25519 server key

[yosuke-wolfssl]

This PR adds kex integration test and sample keys for ed25519 use-case.
Also, this updates echoserver to load the ed25519 keys.

[Copilot]

Pull request overview

Adds Ed25519 host-key coverage to the integration KEX test flow by introducing sample Ed25519 server keys and teaching echoserver to load/use them.

Changes:

• Added an Ed25519 host-key integration test in tests/kex.c.
• Added Ed25519 server key material (.der / .pem) under keys/.
• Updated examples/echoserver/echoserver.c (and embedded cert buffers) to load the Ed25519 private key.

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
wolfssh/certs_test.h	Adds embedded Ed25519 DER key buffer for NO_FILESYSTEM builds.
tests/kex.c	Adds Ed25519 host-key test and adjusts test availability macros.
keys/server-key-ed25519.pem	Adds sample Ed25519 private key (PEM).
keys/server-key-ed25519.der	Adds sample Ed25519 private key (DER).
examples/echoserver/echoserver.c	Adds Ed25519 key loading logic and registers it with the server context.

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