fix(cpworker): reject content after the top-level value in config JSON

[timmy21]

Problem

cjson_parse() called cJSON_ParseWithOpts(json, &end, false). With require_null_terminated=false, cJSON stops after the first complete top-level value and ignores the rest. A config with trailing content was accepted and read only up to that value:

• {"tasks": []}, (stray comma) was accepted as a config with zero tasks.
• A file holding two concatenated configs ran the first one.
• A truncated or doubled write that still started with a complete object passed without an error.

cjson_parse() is used for the config file, the tasks file and parse_config_data(), so all three were affected.

Change

• cjson_parse() passes require_null_terminated=true. Only whitespace (including \r\n) may follow the top-level value.
• The existing error path reports the position of the trailing content, for example JSON parse error at line 1, column 14 near ','.

Not changed, as discussed in #238:

• Duplicate keys (cJSON takes the first) and invalid \uXXXX escapes. The config is generated by cpdaemon with encoding/json, which produces neither.
• The control socket. It parses one newline-framed line at a time since fix(cpworker): serve concurrent and split control-socket requests #304.

Tests

New cases in cpworker/tests/unit/config_file.c:

• test_trailing_garbage_is_rejected: stray comma, exact error message.
• test_second_top_level_value_is_rejected: two concatenated configs.
• test_trailing_garbage_in_config_file_is_rejected: through parse_config_file and parse_tasks_file.
• test_trailing_whitespace_is_accepted: spaces, tabs and CRLF after the value still parse.

The first three fail before the change. All 12 ctest targets and the integration suite (run_test.sh all) pass. Every JSON file under cpworker/ and cpdaemon/ still parses, except the intentionally broken testdata/reload/broken.json.

Fixes #238