🐛 bug: Make idempotency MemoryLock safe for zero-value use

[Copilot]

Description

idempotency.MemoryLock panicked when used as a zero value because its internal keys map was nil on first Lock(). This change makes zero-value usage safe without changing the existing constructor-based path.

• Zero-value safety

  • Lazily initialize MemoryLock.keys inside Lock() before first use.
  • Preserve existing lock lifecycle and cleanup behavior for constructed and zero-value instances.

• Regression coverage

  • Add focused test coverage for:
    • var l idempotency.MemoryLock
    • first Lock() / Unlock()
    • unlocking a missing key after zero-value initialization

• Usage example

  var l idempotency.MemoryLock
  
  if err := l.Lock("key"); err != nil {
      return err
  }
  defer l.Unlock("key")

Changes introduced

• Benchmarks: No benchmark changes.
• Documentation Update: No documentation changes required.
• Changelog/What's New: Zero-value MemoryLock no longer panics on first use.
• Migration Guide: No migration needed.
• API Alignment with Express: Not applicable.
• API Longevity: Preserves the exported type and constructor while making the zero value safe, reducing accidental misuse without API changes.
• Examples: Added a zero-value usage example in this description.

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• Enhancement (improvement to existing features and functionality)
• Documentation update (changes to documentation)
• Performance improvement (non-breaking change which improves efficiency)
• Code consistency (non-breaking change which improves code reliability and robustness)

Checklist

Before you submit your pull request, please make sure you meet these requirements:

• Followed the inspiration of the Express.js framework for new functionalities, making them similar in usage.
• Conducted a self-review of the code and provided comments for complex or critical parts.
• Updated the documentation in the /docs/ directory for Fiber's documentation.
• Added or updated unit tests to validate the effectiveness of the changes or new features.
• Ensured that new and existing unit tests pass locally with the changes.
• Verified that any new dependencies are essential and have been agreed upon by the maintainers/community.
• Aimed for optimal performance with minimal allocations in the new code.
• Provided benchmarks for the new code to analyze and improve upon.

Commit formatting

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[Copilot]

Pull request overview

This PR makes middleware/idempotency.MemoryLock safe to use as a zero value by lazily initializing its internal key map on first lock acquisition.

Changes:

• Lazily initializes MemoryLock.keys inside Lock().
• Adds regression coverage for zero-value MemoryLock lock/unlock behavior and missing-key unlock after initialization.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
middleware/idempotency/locker.go	Adds lazy map initialization to support zero-value MemoryLock usage.
middleware/idempotency/locker_test.go	Adds a zero-value regression subtest for MemoryLock.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.35%. Comparing base (e5e93e6) to head (b4476cb).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4371      +/-   ##
==========================================
- Coverage   91.39%   91.35%   -0.05%     
==========================================
  Files         132      132              
  Lines       13098    13100       +2     
==========================================
- Hits        11971    11967       -4     
- Misses        710      715       +5     
- Partials      417      418       +1     

Flag	Coverage Δ
unittests	91.35% <100.00%> (-0.05%)	⬇️

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