fix: stringify formatter results in nested values, changelog v26.4.2

Follow-up to #2447: a formatter in the nesting chain ($t(key, myFormat))
can return a non-string, and regexSafe would then throw. The value is now
passed through makeString before the $ escaping.

Added t()-level tests for $& in a nested value and in a variable forwarded
through nesting options (the cases that used to hang), plus tests for a
formatter returning a number.