Allow IncomingRequest.req to be overwritten.

[paperdave]

What does this PR do?

Closes #4141

• Documentation or TypeScript types (it's okay to leave the rest blank in this case)
• Code changes

How did you verify your code works?

• Added new test
• Verified manually server.js works

[github-actions]

❌ @paperdave 1 files with test failures on linux-x64:

• test/js/bun/test/test-test.test.ts

View test output

^{#19e6f1156af1b1059e84f3d1c13675030cebcc13}

[github-actions]

❌ @paperdave 2 files with test failures on linux-x64-baseline:

• test/js/bun/test/test-test.test.ts
• test/js/web/fetch/fetch.test.ts

View test output

^{#19e6f1156af1b1059e84f3d1c13675030cebcc13}

[github-actions]

❌ @paperdave 4 files with test failures on bun-darwin-aarch64:

• test/js/bun/spawn/spawn.test.ts
• test/js/bun/test/test-test.test.ts
• test/js/node/dns/node-dns.test.js
• test/js/node/watch/fs.watch.test.ts

View test output

^{#19e6f1156af1b1059e84f3d1c13675030cebcc13}

[github-actions]

❌ @paperdave 7 files with test failures on bun-darwin-x64-baseline:

• test/js/bun/spawn/spawn-streaming-stdin.test.ts
• test/js/bun/spawn/spawn.test.ts
• test/js/bun/sqlite/sqlite.test.js
• test/js/bun/test/test-test.test.ts
• test/js/bun/util/bun-isMainThread.test.js
• test/js/third_party/webpack/webpack.test.ts
• test/js/web/timers/setTimeout.test.js

View test output

^{#19e6f1156af1b1059e84f3d1c13675030cebcc13}

[Jarred-Sumner]

fix the test