Fix subset assertions panicking on incompatible map keys

[Companion]

Fix

Map comparisons with different key types can panic instead of reporting an assertion result

These calls now return false and true respectively without panicking

assert.Subset(t, map[string]int{"1": 1}, map[int]int{1: 1})
assert.NotSubset(t, map[string]int{"1": 1}, map[int]int{1: 1})

Treat incompatible keys as missing and keep matching interface keys working without numeric or named type conversions

Tests

• Reproduced seven regression cases panicking in both assertions before the fix
• All 16 new cases and existing subset tests pass
• Full race suite run with only four Windows symlink permission failures also reproduced on the unchanged base
• All assertion tests pass with race detection when those four filesystem tests are excluded
• Vet formatting and generation checks pass