fix: send BranchResult on failure to prevent channel orphaning

[l33t0]

Summary

When branch.run() returns an error, the spawned task logged the failure but never sent a ProcessEvent::BranchResult back to the channel. This caused the channel to become permanently stuck:

1. Branch ID stays in active_branches indefinitely (leaked handle)
2. Retrigger never fires (should_retrigger stays false)
3. Channel LLM never runs again — no reply is ever delivered
4. Metrics counter never decrements

The fix sends a BranchResult with the error as the conclusion on failure, matching how workers already handle this correctly via WorkerComplete with notify: true.

Root cause

In channel_dispatch.rs, the tokio::spawn block for branch.run() only handled Err with a tracing::error! log — no event was sent. Meanwhile, branch.run() itself only sends BranchResult on the success path (after the agentic loop breaks normally). Any hard failure (e.g. LLM API error, invalid model) fell through silently.

Comparison with workers

Workers already handle this correctly — their spawn block matches on both Ok and Err, always setting notify: true:

let (result_text, notify) = match future.await {
    Ok(text) => (text, true),
    Err(error) => {
        tracing::error!(...);
        (format!("Worker failed: {error}"), true)  // still notifies
    }
};

Test plan

• cargo clippy -- -D warnings clean
• cargo fmt -- --check clean
• Verified single call site for branch.run() — no other spawn locations

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between f0ccd6e and 4b8c591.

📒 Files selected for processing (1)

• src/agent/channel_dispatch.rs

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Walkthrough

Adds error handling for spawned branch tasks by capturing task metadata (event_tx, agent_id, channel_id) before execution and sending a ProcessEvent::BranchResult with failure details when the branch task errors, ensuring failure notifications are reported to the channel.

Changes

Cohort / File(s)	Summary
Branch task failure notification src/agent/channel_dispatch.rs	Captures task metadata and sends ProcessEvent::BranchResult on branch execution errors instead of silently ignoring failures. No public API changes.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely summarizes the main change: fixing a bug where branch failures don't send notifications, causing the channel to become stuck.
Description check	✅ Passed	The description is directly related to the changeset, providing clear context about the bug, root cause, comparison with workers, and test plan.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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