test : add unit tests for metricsCacheKey param serialization edge cases

[tmdeveloper007]

Closes #826.

Summary of What Has Been Done:
Added edge case tests for metricsCacheKey param serialization to the existing metrics-cache.test.ts.

Changes Made:
Modified: test/metrics-cache.test.ts

Added tests:

• null param values are correctly filtered out (not included in key)
• undefined param values are correctly filtered out
• numeric zero (0) is included as '0' in key
• empty string is included as '' in key

Impact it Made:
Validates edge case param serialization. Catches any future regression in null/undefined filtering.

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[github-actions]

GSSoC Label Checklist 🏷️

@Priyanshu-byte-coder — please apply the appropriate labels before merging:

Difficulty (pick one):

• level:beginner — 20 pts
• level:intermediate — 35 pts
• level:advanced — 55 pts
• level:critical — 80 pts

Quality (optional):

• quality:clean — ×1.2 multiplier
• quality:exceptional — ×1.5 multiplier

Validation (required to score):

• gssoc:approved — counts for points
• gssoc:invalid / gssoc:spam / gssoc:ai-slop — does not score

Type labels (type:*) are auto-detected from files and title. Review and adjust if needed.
Points formula: (difficulty × quality_multiplier) + type_bonus

[Priyanshu-byte-coder]

Import pattern is correct — function imported from source. Three infrastructure issues to fix before merge:

1. Missing "test": "vitest run" in package.json scripts — tests can't run in CI without this.
2. vitest.config.ts missing resolve.alias — @/lib/* path aliases fail without:

import path from 'path'
export default defineConfig({
  resolve: { alias: { '@': path.resolve(__dirname, 'src') } },
  test: { globals: true }
})

3. Missing EOF newline on test file.

[tmdeveloper007]

This pull request is fully up-to-date with the latest upstream merges, all review items are addressed, local tests are passing cleanly, and it is fully ready to be merged! 🚀