tests: lib: json: fix test expectation

Update the test_json_decoding test to expect correct unescaped values
based on descriptor type semantics:

- JSON_TOK_STRING: Stores pointer to original JSON string, preserving
  escape sequences as they appear in the source. Used when the original
  escaped format needs to be maintained.

- JSON_TOK_STRING_BUF: Copies and unescapes the string into a fixed-size
  buffer, converting JSON escape sequences (\n, \t, \", etc.) to their
  actual character equivalents. Used when the processed, unescaped string
  is needed.

The test was incorrectly expecting the nested_string_buf field (which
uses JSON_TOK_STRING_BUF) to contain the escaped form "esc: \\t". With
the proper unescaping implementation, this field should contain the
unescaped version "esc: \t" (with actual tab character).

This aligns with the documented behavior where JSON_TOK_STRING_BUF
performs unescaping during the copy operation, while JSON_TOK_STRING
simply references the original escaped string.

Signed-off-by: BUDKE Gerson Fernando <gerson.budke@leica-geosystems.com>

Author: nandojve

tests/lib/json/src/main.c

@@ -554,8 +554,7 @@ ZTEST(lib_json_test, test_json_decoding)
 	zassert_str_equal(ts.some_nested_struct.nested_string,
 			  "this should be escaped: \\t",
 			  "Nested string not decoded correctly");
-	zassert_str_equal(ts.some_nested_struct.nested_string_buf,
-			  "esc: \\t",
+	zassert_str_equal(ts.some_nested_struct.nested_string_buf, "esc: \t",
 			  "Nested string-array not decoded correctly");
 	zassert_equal(ts.some_array_len, 5,
 		      "Array doesn't have correct number of items");