anchor the whole YANG pattern in fixYangRegexp

[nabhan06]

fixYangRegexp only wraps the converted pattern in parens when the YANG pattern does not already start with a caret, so ^ab|cd$ is passed to the regexp package unchanged and evaluated as (^ab)|(cd$). Since YANG patterns are implicitly anchored to the whole string, ValidateStringRestrictions then accepts values the schema forbids, e.g. "abcdef" for a leaf restricted to ^ab|cd$.

The same first/last character inspection also treats an escaped trailing dollar as an anchor: a pattern like [0-9]+\$ becomes ^([0-9]+\)$, which does not compile, so every value for that leaf fails validation with a regexp parse error.

This strips whatever anchors the pattern already carries (a trailing $ counts as an anchor only when it is unescaped) and then always wraps the remainder in ^(...)$. Existing expectations for ^abc and ^abc$ change to ^(abc)$, which is equivalent.