fix: align std.assertEqual failure messages

[He-Pin]

Motivation:
std.assertEqual is defined by the official stdlib as true when a == b, otherwise an error whose message starts with Assertion failed. . The stdlib also escapes string operands for display and relies on Jsonnet stringification for non-string operands.

sjsonnet's native builtin skipped the official prefix and rendered failure operands through ujson.Value.toString, so object and array diffs were compact JSON such as {"x":1} instead of Jsonnet-rendered values such as {"x": 1}.

Modification:

• Keep the existing success fast path through ev.equal(a, b) before materializing either operand.
• Render failure operands with Materializer.stringify, matching Jsonnet value rendering for objects, arrays, strings, numbers, booleans, and null.
• Change the builtin failure text to Assertion failed. <lhs> != <rhs>.
• Update the existing upstream/go golden files and add a focused new_test_suite regression for object rendering in the failure message.

Result:

Case	Official std.jsonnet / C++ jsonnet	go-jsonnet	jrsonnet	sjsonnet before	sjsonnet after
std.assertEqual(1, 2)	Assertion failed. 1 != 2	Assertion failed. 1 != 2	Assertion failed. 1 != 2	[std.assertEqual] 1 != 2	[std.assertEqual] Assertion failed. 1 != 2
std.assertEqual({x: 1}, {x: 2})	Assertion failed. {"x": 1} != {"x": 2}	Assertion failed. {"x": 1} != {"x": 2}	Assertion failed. {"x": 1} != {"x": 2}	[std.assertEqual] {"x":1} != {"x":2}	[std.assertEqual] Assertion failed. {"x": 1} != {"x": 2}
std.assertEqual("\n ", "\n")	Assertion failed. "\n " != "\n"	Assertion failed. "\n " != "\n"	Assertion failed. with raw newline text	[std.assertEqual] "\n " != "\n"	[std.assertEqual] Assertion failed. "\n " != "\n"
std.assertEqual({x:: 42}, {})	true	true	true	true	true
std.assertEqual(function() 1, function() 2)	equality error before assertion diff	equality error before assertion diff	equality error before assertion diff	equality error before assertion diff	equality error before assertion diff

sjsonnet still keeps its existing formatted builtin frame prefix ([std.assertEqual]) around native builtin errors; this PR intentionally avoids a broad error-format change.

References:

• https://jsonnet.org/ref/stdlib.html#std-assertEqual
• https://github.com/google/jsonnet/blob/9fdb2b9653be6b2cfc6eb012a7915679ba9447cd/stdlib/std.jsonnet#L853-L861