feat: add solutions for lc No.3120,3121

[yanglbme]

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[Copilot]

Pull request overview

This PR updates the existing multi-language solution for LC 3121 by simplifying the per-letter validation logic, and adds Rust implementations (plus documentation snippets) for LC 3120 and LC 3121 within the repository’s solutions collection.

Changes:

• Simplified the “special character” check for LC 3121 across C++/Go/Java/TypeScript (removing redundant first[...] > 0 presence checks).
• Added Rust solutions for LC 3120 and LC 3121.
• Updated both Chinese and English READMEs to include the Rust implementations and keep code snippets in sync with the solution files.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
solution/3100-3199/3121.Count the Number of Special Characters II/Solution.ts	Refactors per-letter check for readability by introducing a/b and removing redundant presence test.
solution/3100-3199/3121.Count the Number of Special Characters II/Solution.rs	Adds Rust implementation for LC 3121.
solution/3100-3199/3121.Count the Number of Special Characters II/Solution.java	Refactors per-letter check with a/b and removes redundant presence test.
solution/3100-3199/3121.Count the Number of Special Characters II/Solution.go	Simplifies condition by removing redundant first[...] > 0 check.
solution/3100-3199/3121.Count the Number of Special Characters II/Solution.cpp	Simplifies condition by removing redundant first[...] truthiness check.
solution/3100-3199/3121.Count the Number of Special Characters II/README.md	Syncs snippet changes and adds Rust section.
solution/3100-3199/3121.Count the Number of Special Characters II/README_EN.md	Syncs snippet changes and adds Rust section.
solution/3100-3199/3120.Count the Number of Special Characters I/Solution.rs	Adds Rust implementation for LC 3120.
solution/3100-3199/3120.Count the Number of Special Characters I/README.md	Adds Rust snippet for LC 3120.
solution/3100-3199/3120.Count the Number of Special Characters I/README_EN.md	Adds Rust snippet for LC 3120.

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