fix(tun): check tun exist before setting IP to preserve pre-configured routes

[ibigbug]

🤔 This is a ...

• Bug fix

🔗 Related issue link

close #939

💡 Background and solution

When TUN interface already exists (e.g., pre-configured by system administrator), setting IP addresses can overwrite existing network routes and configurations. This change checks if the TUN device exists before attempting to configure its IP addresses, preserving any pre-existing network configuration.

The solution:

1. Check if the TUN interface already exists using network_interface crate
2. Only set IPv4/IPv6 addresses if the interface doesn't exist
3. Log appropriate messages to indicate whether creating new or using existing interface

📝 Changelog

• fix(tun): Preserve pre-configured TUN interface routes by checking existence before IP configuration
• refactor: Split TUN inbound handlers into separate modules for better code organization

☑️ Self-Check before Merge

• Doc is updated/provided or not needed
• Changelog is provided or not needed

🤖 Generated with Claude Code

[Copilot]

Pull Request Overview

This PR fixes an issue where setting IP addresses on an existing TUN interface could overwrite pre-configured network routes. The solution checks if the TUN device already exists before attempting to configure its IP addresses, preserving any existing network configuration.

Key changes:

• Check TUN interface existence using network_interface crate before IP configuration
• Only set IPv4/IPv6 addresses if the interface doesn't exist
• Refactor TUN inbound handlers into separate modules for better organization

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
clash-lib/src/proxy/tun/mod.rs	Add new stream module export
clash-lib/src/proxy/tun/inbound.rs	Add TUN existence check logic and remove inline handler functions
clash-lib/src/proxy/tun/stream.rs	New module containing extracted TCP stream handler
clash-lib/src/proxy/tun/datagram.rs	New module containing extracted UDP datagram handler

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[codecov]

Codecov Report

❌ Patch coverage is 1.32450% with 149 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
clash-lib/src/proxy/tun/datagram.rs	0.00%	109 Missing ⚠️
clash-lib/src/proxy/tun/stream.rs	0.00%	22 Missing ⚠️
clash-lib/src/proxy/tun/inbound.rs	0.00%	18 Missing ⚠️

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