Use fixed-size span for Ed25519 pubkey conversion

[Ap4sh]

summary

compute_x25519_pubkey() now requires a 32-byte view, so dynamic spans with the wrong size cannot reach libsodium through this API accidentally

file server keys are parsed as fixed-size ed25519_pubkey values before conversion, rejecting oversized values that were previously accepted with trailing bytes ignored

this intentionally changes the C++ helper signature to match the fixed-size crypto input pattern already used elsewhere in libsession

tests

• ./utils/format.sh verify
• release build with GCC 15.2 and WARNINGS_AS_ERRORS=ON
• ./Build/tests/testAll "[backend][session_file_server]"
• ./Build/tests/testAll
• ./Build/tests/testLogging
• release build with Clang 19 and WARNINGS_AS_ERRORS=ON
• ./Build-clang/tests/testAll "[backend][session_file_server]"
• ./Build-clang/tests/testAll
• ./Build-clang/tests/testLogging