Normalize invalid FileAppender buffer sizes

[metsw24-max]

Normalize invalid FileAppender buffer size values before storing them in the internal int buffer size field.

FileAppender::_priv->bufferSize is stored as an int, but multiple code paths accepted larger or negative values without validation. On 64-bit systems, oversized values such as 2G could overflow during narrowing conversion and later expand into extremely large size_t values during buffered writer initialization, preventing flush thresholds from triggering and causing unbounded memory growth.

Changes Made

• Added validation for BUFFERSIZE values parsed via OptionConverter::toFileSize().

• Invalid values (negative or greater than INT_MAX) now:

  • emit a LogLog::warn
  • fall back to the default 8 KiB buffer size.

• Added bounds checking in setFileInternal() before narrowing size_t to int.

• Updated setBufferSize(int) to normalize negative inputs to 0 with a warning.

• Added <limits> for safe integer boundary checks.

Tests Added

Added regression tests covering:

• Negative BUFFERSIZE option values.
• Oversized BUFFERSIZE option values (2G overflow path).
• Direct negative setBufferSize(int) calls.

Each test verifies that the resulting buffer size remains non-negative and does not pass through signed/unsigned reinterpretation paths.