fix(test): widen gateway-internal-mcp HMAC timestamp offsets to 60s (#4015)

The "31s in the future" test signs ts = floor(now)+31 and asserts the
validator rejects it (offset > 30s window). The validator rejects only
when |now - ts| > 30, so an offset of exactly 30 is accepted. On a busy
CI runner a single wall-clock second can elapse between the test's
Date.now() and the validator's, collapsing the perceived offset to
exactly 30 -> 200 instead of 401 -> intermittent flake.

Widen both boundary offsets to 60s (30s clear of the window edge) so no
realistic scheduling jitter can reach the inclusive-accept boundary.
The -31 past test cannot flake (rounding only pushes it further out) but
is moved to -60 for symmetry. Test names updated to match.

Closes #3848