feat(path): add Rust convert_file_src function

[ddmoney420]

Summary

Adds a Rust equivalent of the JavaScript convertFileSrc() function, allowing backend code to convert device file paths to URLs that can be loaded by the webview.

Motivation

This is useful when processing file paths in Rust (e.g., markdown parsers, image processors) that need to generate URLs for the webview to load assets. Currently this requires hacky workarounds like calling JavaScript from Rust or manually implementing the URL transformation.

Implementation

The function in tauri::path::convert_file_src:

• URL encodes the file path using percent_encoding
• Uses platform-appropriate scheme (http:// on Windows/Android, custom protocol on other platforms)
• Supports custom protocols via optional parameter
• Matches the behavior of the JavaScript convertFileSrc() exactly

Usage

use tauri::path::convert_file_src;

// Default "asset" protocol
let url = convert_file_src("/path/to/video.mp4", None);
// Returns: "asset://localhost/%2Fpath%2Fto%2Fvideo.mp4" (on macOS/Linux)
// Returns: "http://asset.localhost/%2Fpath%2Fto%2Fvideo.mp4" (on Windows/Android)

// Custom protocol
let url = convert_file_src("/path/to/file", Some("custom"));

Test plan

• Added unit tests for default protocol, custom protocol, and special character handling
• Tests verify platform-specific URL schemes

Fixes #12022

🤖 Generated with Claude Code

[FabianLars]

Thanks for the PR but just like #14816 i'll close it in favor of #14786.
Funny that all 3 PRs made the same mistake of missing the https option though 😅