fix: surface actual actor failure instead of generic recv error in collector handles

[rodonile]

Report the real cause when a collector actor fails to start, instead of
collapsing it into a generic "receiving response from actor" error.

Reason

FlowCollectorActorHandle::new and ActorHandle::new send an initial
LocalAddr command to the freshly spawned actor task and await its
reply to learn the bound local address. If the actor fails early (e.g.
a UDP socket bind error) it exits and drops the command receiver /
reply sender, which previously surfaced to the caller only as a
generic SendError/ReceiveError — the actual bind error (or panic)
was silently discarded.

Changes

• FlowCollectorActorError / UdpNotifActorError now implement
  Error::source (replacing the deprecated Error::cause), chaining
  the underlying I/O errors.
• FlowCollectorActorHandleError / ActorHandleError gain
  ActorFailed(_), ActorPanicked(_), and ActorExited variants
  carrying the real cause, in place of the old generic ReceiveError.
• FlowCollectorActorHandle::new / ActorHandle::new race the initial
  command send and reply recv against the actor's
  JoinHandle (tokio::select! { biased; ... }) so an early actor failure,
  panic, or clean early exit is detected and reported precisely instead
  of appearing as a channel error.

[Copilot]

🟡 Changes recommended

The new early-failure surfacing still leaves a race where send(LocalAddr) failures return generic SendError (and the new ActorFailed variants aren’t exposed via the error chain), which can continue to hide the real startup cause.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

This PR improves startup error reporting for flow-service and udp-notif-service actor handles by detecting when an actor task terminates before it can return its LocalAddr, and by propagating the actor’s real failure (e.g., socket bind errors) instead of a generic channel receive error.

Changes:

• Added ActorFailed(...) variants to the handle error enums to carry the actor’s startup failure.
• Updated *_Handle::new() to tokio::select! between the actor JoinHandle and the LocalAddr reply to detect early termination and surface the underlying error.

File summaries

File	Description
crates/udp-notif-service/src/actor.rs	Adds ActorFailed(UdpNotifActorError) and races join-handle vs. local-addr response during handle creation.
crates/flow-service/src/flow_actor.rs	Adds ActorFailed(FlowCollectorActorError) and applies the same early-exit detection during handle creation.

Review details

Suppressed comments (2)

crates/udp-notif-service/src/actor.rs:1045

• If the actor exits very quickly (e.g. bind() fails), cmd_tx.send(LocalAddr) can also fail with the receiver already dropped, and this path still returns SendError which hides the real cause. Since this PR is about surfacing early actor startup failures, consider awaiting the join handle on send failure and returning ActorFailed when available.

        cmd_tx
            .send(ActorCommand::LocalAddr(tx))
            .await
            .map_err(|_| ActorHandleError::SendError)?;

crates/flow-service/src/flow_actor.rs:967

• As with the udp-notif actor, if the actor task fails immediately (e.g. socket bind failure), cmd_tx.send(LocalAddr) can race and fail with the receiver already dropped. Returning SendError here still hides the actor's real startup error; awaiting the join handle on send failure would better match the PR goal of surfacing early-exit failures.

        cmd_tx
            .send(FlowCollectorActorCommand::LocalAddr(tx))
            .await
            .map_err(|_| FlowCollectorActorHandleError::SendError)?;

• Files reviewed: 2/2 changed files
• Comments generated: 2
• Review effort level: Lite

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[Copilot]

🟡 Changes recommended

Both handle error types now include non-Copy payloads but still match *self in Error::description(), which will not compile due to moving out of &self.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 2
• Review effort level: Lite

[Copilot]

🟡 Changes recommended

The new source() wiring still won’t expose underlying I/O errors (and still masks join/panic failures) unless the actor error types also implement source() and join failures are represented more precisely than ReceiveError/SendError.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 4
• Review effort level: Lite

[Copilot]

🟡 Changes recommended

The new startup failure modes and error variants aren’t covered by tests despite existing in-file test modules, increasing regression risk for this critical error-reporting behavior.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 2
• Review effort level: Lite