PS-10347 [8.0]: Ensure strict JSON compliance and robust field separation in audit_log_read()/AuditJsonHandler

[inikep]

1. Refactors the JSON serialization logic within AuditJsonHandler to guarantee strict compliance with the JSON standard by eliminating trailing commas and centralizing field separation control.

The previous approach of appending ", " after every value handler was inconsistent and required error-prone comma removal logic in EndObject.

This change adopts a safer, state-driven approach:

• Centralized Comma Management: The responsibility for adding the comma separator is moved entirely from the value handlers (Int, String, etc.) to the Key() handler.
• State-Driven Separation: The new m_is_first_field state flag, set in StartObject() and checked/updated in Key(), ensures a comma is prepended only when necessary (i.e., not for the first field), thereby naturally preventing trailing commas within objects.
• Inter-Event Separation: Confirmed that the ,\n separator is correctly appended to separate top-level audit event objects in the array.

2. The audit_log_read() UDF was failing to respect the max_array_length parameter, returning all records instead of the specified limit. This was caused by the read loop not checking the is_batch_end flag.

Additionally, attempting to read the remaining records in a subsequent call caused an infinite loop or parsing errors. This occurred because a new rapidjson::Reader was created for each call, losing the internal state required to resume parsing mid-stream (e.g., handling the comma separator between array elements).

The fix involves:

• Respecting the is_batch_end flag in the AuditLogReader::read loop to stop processing when the limit is reached.
• Storing the rapidjson::Reader instance within AuditLogReaderContext to preserve parsing state across multiple audit_log_read() calls.
• Adding error checking for reader->HasParseError() to prevent infinite loops on malformed data or state mismatches.
• Updating the udf_audit_log_read_validate_output test case to verify correct behavior for max_array_length.