Add bounded queue support for enqueue option

[GangEunzzang]

PR Title

Add bounded queue support for enqueue option (#1419)

Summary

This PR adds support for bounded queues when using enqueue to prevent unbounded memory growth with slow sinks.

Fixes #1419

Changes

New enqueue options:

Usage	Description
enqueue=True	Unbounded queue (existing behavior)
enqueue=100	Bounded queue with max size 100, blocks when full
enqueue={"size": 100}	Same as above
enqueue={"size": 100, "overflow": "block"}	Blocks when full (default)
enqueue={"size": 100, "overflow": "drop"}	Drops messages when full

Overflow policies:

• "block" (default): Blocks logging call until queue has space (backpressure)
• "drop": Silently drops the message when queue is full (non-blocking)

Example

from loguru import logger

# Bounded queue that blocks when full
logger.add("file.log", enqueue=100)

# Bounded queue that drops messages when full
logger.add("file.log", enqueue={"size": 100, "overflow": "drop"})

Test Plan

- Added 4 new test cases for bounded queue functionality
- All 19 existing enqueue tests pass
- Tested on macOS and Linux (Docker)