fix: handle ujson surrogate parse exceptions in JSON imports

[He-Pin]

Motivation

Importing malformed Unicode surrogate escapes from .json files can make ujson throw a plain java.lang.Exception. That previously escaped as an sjsonnet internal error instead of falling back to normal Jsonnet parsing and reporting a parse error.

Modification

• Catch ujson's known bare java.lang.Exception surrogate parse failures in the .json import fast path.
• Keep the catch narrow by requiring the exact exception class plus known surrogate error message prefixes.
• Add regression coverage for low-surrogate, reversed-surrogate, high-before-non-low-escape, high-before-high-escape failures, and valid surrogate pairs.

Result

Case	go-jsonnet v0.22.0	jrsonnet 0.5.0-pre99	sjsonnet before	sjsonnet after
Import JSON containing \uDC00 as a lone low surrogate	Truncated unicode surrogate pair escape sequence in string literal	syntax error: invalid string escape	Internal Error caused by Un-paired low surrogate	sjsonnet.ParseError without Internal Error
Import JSON containing \uDE00\uD83D as a reversed surrogate pair	parse error	syntax error: invalid string escape	Internal Error caused by Un-paired low surrogate	sjsonnet.ParseError without Internal Error

Valid surrogate pairs still use the JSON import fast path.

Risks

• The catch depends on ujson's current exception type and message prefixes for surrogate failures.
• This does not add a separate pre-validation scan for every JSON import.