Support JSON-formatted log entries in log reading tools

[HeathNaylor]

This PR does the following:

• Auto-detect JSON log format in the ReadsLogs trait and split entries by newlines instead of PSR-3 timestamps
• Handle JSON error detection in LastError tool, supporting both numeric level (level >= 400) and string level (level_name = "ERROR") fields
• No behavior change for PSR-3 formatted logs
• Tested against LogstashFormatter and LogglyFormatter in addition to JsonFormatter since these are the other Monolog formatters that write JSON to log files and would hit the same issue

Fixes #275

Test plan

• 7 new tests across ReadLogEntriesTest and LastErrorTest covering JsonFormatter, LogstashFormatter, and LogglyFormatter output formats
• Tests verify correct entry count, error detection, and exclusion of non-error entries for each format