fix: pad ECDSA signature fields to 32 bytes in signMsg to prevent intermittent P256 precompile test failures

[zsystm]

Description

Pad each ECDSA signature field (r, s, X, Y) to 32 bytes in signMsg to eliminate the intermittent “valid signature” test failures caused by un-padded big-int bytes.

Closes: #273

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[cloudgray]

Thanks for fixing such a bothersome problem.

[vladjdk]

LGTM, thanks!

@aljo242 @technicallyty this fixes the flaky test we've been having problems with :)