validate utf-16 surrogate halves in decodeUnicodeCodePoint

[SABITHSAHEB]

1. after a high surrogate (D800-DBFF) the next \u escape is consumed and combined without checking it is a low surrogate, so "\uD801\u0041" or "\uD801\uD801" parse to a wrong astral code point and the second escape's real value is lost.
2. a low surrogate that appears on its own (e.g. "\uDC00") is passed straight through and gets written out as the invalid UTF-8 bytes ED B0 80.
   Validated the low-surrogate range when completing a pair and rejected an unpaired low surrogate, in both Reader and OurReader. Added reader tests for the two new rejection paths; existing valid-pair cases are unaffected.