feat(instructions): add Java JUnit 5 assertions best practices 🤖🤖🤖

[burakkaygusuz]

This PR introduces a comprehensive instruction set for Java JUnit 5 (Jupiter) Assertions, optimized for GitHub Copilot.

🎯 Purpose

To provide GitHub Copilot with clear, technical, and performance-oriented guidelines for generating and refactoring Java test code using modern JUnit 5.8+ features.

🚀 Key Features:

• Performance & Lazy Evaluation: Enforces Supplier<String> for expensive failure messages to avoid overhead in passing tests.
• Modern Jupiter APIs: Explicitly covers assertThrowsExactly (JUnit 5.8+) and assertInstanceOf for better type safety and stricter testing.
• SLA & Deadlines: Provides guidelines for assertTimeout vs assertTimeoutPreemptively with explicit warnings about ThreadLocal propagation.
• Readability & Standards: Standardizes "Expected Value First" ordering and promotes grouping related assertions with assertAll.
• Anti-Pattern Prevention: Actively prevents legacy JUnit 4 imports, suppressing assertions, and using weak boolean checks instead of semantic equality assertions.

🛠️ Verification & Quality:

• Ran npm install and npm run build to update README.instructions.md.
• Verified all technical content against official JUnit 5 documentation.
• Normalized line endings to LF using bash eng/fix-line-endings.sh.
• Followed lowercase-hyphenated naming convention: java-junit5-assertions.instructions.md.

This contribution aligns with the repository'''s standards for high-quality, actionable AI instructions. 🤖🤖🤖