feat: add Rust solution for lc No.3714

[yanglbme]

No description provided.

[Copilot]

Pull request overview

Adds a Rust implementation for LeetCode problem 3714. Longest Balanced Substring II to keep language coverage consistent with the existing multi-language solution set in solution/3700-3799.

Changes:

• Added Solution.rs implementing the same approach as the other language solutions (case split: 1-char / 2-char / 3-char balanced substrings).
• Updated both README.md and README_EN.md to include the Rust solution snippet under the language tabs.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
solution/3700-3799/3714.Longest Balanced Substring II/Solution.rs	New Rust solution implementing prefix-sum + hash map approach for 2/3-character cases and run-length scan for 1-character case.
solution/3700-3799/3714.Longest Balanced Substring II/README.md	Adds Rust code section to the Chinese write-up’s language tabs.
solution/3700-3799/3714.Longest Balanced Substring II/README_EN.md	Adds Rust code section to the English write-up’s language tabs.

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