fix: guard RequestManagerChange token logging against an empty token

[mrofreP]

RequestManagerChange logs the confirmation token as
confirmationToken[:8]+"...". On master, GenerateSecureToken swallows a
crypto/rand failure and returns "", so confirmationToken can be empty
and ""[:8] panics -- a live path, however rare. Guard it with
confirmationToken[:min(len(confirmationToken), 8)], matching the guard
applied to the user-supplied token in ConfirmManagerChange.

The guard is kept even though it is not the root fix: once
GenerateSecureToken propagates its error (companion PR, which should
merge first), the handler returns 500 before this line and the token is
guaranteed non-empty. The guard then costs nothing and future-proofs the
log line against a change in the token source.

Caveat left as-is: the "..." suffix is unconditional, so an empty token
logs just "..." (harmless -- it leaks nothing). Collapsing all six
copies of this masking expression into a shared helper is the right
cleanup but is out of scope here.

[codacy-production]

Up to standards ✅

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