Fix blake2b_final coverity reports

[embhorn]

Description

• OOB access in blake2b_final
• Use memmove instead of memcpy

Testing

Coverity confirmed

Checklist

• added tests
• updated/added doxygen
• updated appropriate READMEs
• Updated manual and documentation

[Copilot]

Pull Request Overview

Fixes an out-of-bounds access in blake2b_final by tightening the buffer-length check and using a safe copy function for overlapping memory regions.

• Changed the boundary condition to reject more than one full block leftover.
• Swapped XMEMCPY for XMEMMOVE to safely handle overlapping buffer regions.

Comments suppressed due to low confidence (2)

wolfcrypt/src/blake2b.c:359

• The new boundary check for leftover bytes isn't covered by existing tests. Add unit tests for cases where buflen is just above and just below BLAKE2B_BLOCKBYTES to ensure correct behavior.

    if ( S->buflen > BLAKE2B_BLOCKBYTES )

wolfcrypt/src/blake2b.c:361

• [nitpick] Please update the function’s doxygen or inline comments to explain why XMEMMOVE is used instead of XMEMCPY, clarifying the need for safe overlapping memory copies.

    XMEMMOVE( S->buf, S->buf + BLAKE2B_BLOCKBYTES, (wolfssl_word)S->buflen );

[embhorn]

Retest this please

Build [wolfSSL » PRB-140-3-tests #5865](https://cloud.wolfssl-test.com/jenkins/job/wolfSSL/job/PRB-140-3-tests/5865/) completed: FAILURE
[Pipeline] }
[wolfSSL » PRB-140-3-tests #5865](https://cloud.wolfssl-test.com/jenkins/job/wolfSSL/job/PRB-140-3-tests/5865/) completed with status FAILURE (propagate: false to ignore)
org.jenkinsci.plugins.workflow.actions.ErrorAction$ErrorId: 3ffca30a-ed4f-48d5-8bdd-d135757929af

java.io.StreamCorruptedException: invalid stream header: 636F7272
	at java.base/java.io.ObjectInputStream.readStreamHeader(ObjectInputStream.java:958)
	at java.base/java.io.ObjectInputStream.<init>(ObjectInputStream.java:392)
	at hudson.remoting.ObjectInputStreamEx.<init>(ObjectInputStreamEx.java:50)
	at hudson.remoting.Command.readFrom(Command.java:141)
	at hudson.remoting.Command.readFrom(Command.java:127)