[codex] Fix dynamic handler test env cleanup

[coe0718]

Summary

Fixes the pre-existing Biome lint failure in dynamic-handlers.test.ts without changing the test semantics.

What Changed

• replaces delete process.env.* cleanup with a small restoreEnvVar() helper
• uses Reflect.deleteProperty() so env vars are actually removed instead of being set to the string "undefined"
• keeps the original env value when one existed before the test
• applies Biome import ordering in the file

Why

Biome flags the delete operator in this test file, but its suggested replacement (process.env.KEY = undefined) is behaviorally unsafe for Node/Bun because env vars become the string "undefined" instead of being removed.

This keeps the lint fix small and behavior-preserving.

Validation

• bash -lc 'export PATH="$HOME/.bun/bin:$PATH" && /home/coemedia/.bun/bin/bun test src/mcp/__tests__/dynamic-handlers.test.ts'
• bun run lint
• ./node_modules/.bin/biome check src/mcp/__tests__/dynamic-handlers.test.ts

[mcheemaa]

Hey @coe0718 - thanks for catching this! Really appreciate you taking the time to put together a fix.

We ended up fixing the same lint issue in v0.18.0 (commit 2bca825, merged via #6), but looking at both approaches more carefully, your Reflect.deleteProperty() pattern is actually more correct than our process.env.X = undefined. Setting undefined on process.env can coerce to the string "undefined" instead of actually removing the key. Your approach properly removes it.

We're going to close this PR since the changes overlap and would conflict, but we'll adopt the Reflect.deleteProperty pattern in a follow-up. Good catch on the semantics.

Your memory PRs (#2, #3, #4) are excellent work - we're reviewing those now. Thanks again!