io_uring: thread cqe->flags through CompletionCb

[aburan28]

Commit Message:
io_uring: thread cqe->flags through CompletionCb

No-behavior-change refactor in preparation for multishot recv. CompletionCb
gains a uint32_t flags argument carrying the raw cqe->flags value from the
kernel. Kernel completions forward cqe->flags; injected completions pass 0.
The worker callback ignores it for now. A follow-up will introduce a buf-ring
plus prepareRecvMultishot and start observing IORING_CQE_F_BUFFER and
IORING_CQE_F_MORE.

Additional Description:
The full multishot-recv change touches the buf-ring lifecycle in IoUringImpl,
the read path in IoUringServerSocket, and the proto config. Threading flags
through the existing callback is mechanical and easy to review on its own;
landing it first lets the rest of the stack be a behavioral change rather than
a behavioral + signature change.

Files touched:

• envoy/common/io/io_uring.h — CompletionCb typedef + doc.
• source/common/io/io_uring_impl.cc — pass cqe->flags for kernel
  completions, 0 for injected.
• source/common/io/io_uring_worker_impl.cc — accept the new arg in the
  worker lambda (unused for now).
• test/common/io/io_uring_impl_test.cc,
  test/common/io/io_uring_worker_impl_test.cc — update test lambdas.

IoUringSocket::on* virtual methods are intentionally unchanged here; only
onRead will need flags, in the multishot recv change.

AI usage disclosure: Portions of the code and/or PR description were drafted
with the assistance of Claude (Anthropic). I reviewed and understand all
submitted code.

Risk Level: Low
(No behavior change; mechanical signature change to an internal callback.
Worker still discards the new argument.)

Testing: Existing io_uring unit and integration tests on Linux CI.

Docs Changes: N/A

Release Notes: N/A (internal refactor, no user-visible or extension-developer-
visible change.)

Platform Specific Features: N/A. io_uring is Linux-only and the affected code
already lives behind the existing io_uring build gating; this PR does not
change platform support or introduce new platform-specific surface.

Runtime guard: N/A (no behavioral change).

[repokitteh-read-only]

Hi @aburan28, welcome and thank you for your contribution.

We will try to review your Pull Request as quickly as possible.

In the meantime, please take a look at the contribution guidelines if you have not done so already.

🐱

Caused by: #44667 was opened by aburan28.

see: more, trace.

[repokitteh-read-only]

As a reminder, PRs marked as draft will not be automatically assigned reviewers,
or be handled by maintainer-oncall triage.

Please mark your PR as ready when you want it to be reviewed!

🐱

Caused by: #44667 was opened by aburan28.

see: more, trace.

[zuercher]

Please add the fields from the pull request template to your PR description and review CONTRIBUTING.md, especially the section on AI use.

[zuercher]

/wait