Optimize memory allocation logic when constructing packet buffer

[Searchstars]

Changed the vector size used to construct the packet buffer from u16::MAX as usize + offset to mtu + offset, reducing unnecessary memory allocations (especially on memory-constrained microcontroller devices).

[Copilot]

Pull Request Overview

This PR optimizes memory allocation for packet buffer construction in resource-constrained environments (especially microcontrollers running ESP-IDF). The change reduces the buffer size from u16::MAX + offset (≈65KB) to mtu + offset, using the MTU configuration value with a minimum of 1280 bytes.

• Replaces hardcoded u16::MAX buffer size with MTU-based allocation
• Adds MIN_IP_PACKET_LEN constant (1280 bytes) per RFC 8200 for IPv6
• Extends target OS support to include ESP-IDF

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[Searchstars]

The reviewer’s comments have been addressed. The primary update changes the default MTU from u16::MAX to the RFC-defined theoretical minimum to reduce unnecessary memory usage, while still allowing users to specify a larger MTU if required.