fix(ReadableStream) flush as much we can before ending the stream

[cirospaciari]

What does this PR do?

Fix: https://github.com/oven-sh/bun/issues/15235

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

How did you verify your code works?

Test

[robobun]

:x: @cirospaciari, your commit 94e36557bebea7c948e58f172f3d774f91fcc846 has 12 failures in #6605:

test/integration/next-pages/test/dev-server.test.ts - 1 failing on 🐧 3.20 x64-baseline

test/integration/next-pages/test/dev-server.test.ts - 1 failing on 🐧 3.20 x64

test/regression/issue/ctrl-c.test.ts - 1 failing on 🐧 22.04 aarch64

test/js/bun/http/serve.test.ts - segmentation fault on 🐧 3.20 aarch64

test/js/bun/http/serve.test.ts - segmentation fault on 🐧 3.20 x64-baseline

test/js/bun/http/serve.test.ts - segmentation fault on 🐧 3.20 x64

test/js/bun/http/serve.test.ts - 1 failing on 🍎 14 x64

test/v8/v8.test.ts - 22 failing on 🐧 3.20 aarch64

test/v8/v8.test.ts - 22 failing on 🐧 3.20 x64-baseline

test/v8/v8.test.ts - 22 failing on 🐧 3.20 x64

test/js/deno/crypto/webcrypto.test.ts - 1 failing on 🐧 12 aarch64

test/js/deno/performance/performance.test.ts - 1 failing on 🐧 22.04 aarch64

test/cli/install/registry/bun-install-registry.test.ts - timeout on 🐧 22.04 x64

test/cli/install/registry/bun-install-registry.test.ts - timeout on 🐧 22.04 aarch64

test/js/node/child_process/child_process.test.ts - 1 failing on 🐧 3.20 aarch64

test/js/node/child_process/child_process.test.ts - 1 failing on 🐧 3.20 x64-baseline

test/js/node/child_process/child_process.test.ts - 1 failing on 🐧 3.20 x64

test/cli/install/bun-link.test.ts - 4 failing on 🐧 3.20 aarch64

test/cli/install/bun-link.test.ts - 4 failing on 🐧 3.20 x64-baseline

test/cli/install/bun-link.test.ts - 4 failing on 🐧 3.20 x64

test/js/node/test/parallel/fs-watch-recursive-linux-parallel-remove.test.js - 1 failing on 🐧 12 x64-baseline

test/js/node/test/parallel/fs-watch-recursive-linux-parallel-remove.test.js - 1 failing on 🐧 11 x64-baseline

test/js/node/test/parallel/fs-watch-recursive-linux-parallel-remove.test.js - 1 failing on 🐧 20.04 x64-baseline

test/js/node/test/parallel/fs-watch-recursive-linux-parallel-remove.test.js - 1 failing on 🐧 11 x64

test/js/node/test/parallel/fs-watch-recursive-linux-parallel-remove.test.js - 1 failing on 🐧 12 x64

test/js/node/test/parallel/fs-watch-recursive-linux-parallel-remove.test.js - 1 failing on 🐧 22.04 x64

test/js/node/test/parallel/fs-watch-recursive-linux-parallel-remove.test.js - 1 failing on 🐧 11 aarch64

test/js/node/test/parallel/fs-watch-recursive-linux-parallel-remove.test.js - 1 failing on 🐧 20.04 x64

test/js/node/test/parallel/fs-watch-recursive-linux-parallel-remove.test.js - 1 failing on 🐧 22.04 aarch64

test/js/node/test/parallel/fs-watch-recursive-linux-parallel-remove.test.js - 1 failing on 🐧 3.20 x64-baseline

test/js/node/test/parallel/fs-watch-recursive-linux-parallel-remove.test.js - 1 failing on 🐧 3.20 x64

test/js/bun/util/sleep.test.ts - 1 failing on 🪟 2019 x64

test/js/node/dns/node-dns.test.js - 1 failing on 🪟 2019 x64-baseline