fix(core): make JobResult.Finish idempotent

[walcz-de]

Problem

JobResult.Finish unconditionally closes the ready channel. A job can legitimately be finished from more than one code path — an early finish inside a tool decision, followed by the regular completion of the job. The second call hits close(j.ready) on an already closed channel and panics with close of closed channel, which brings the whole agent down.

Reproduction

j := types.NewJobResult()
j.Finish(errors.New("first"))
j.Finish(nil) // panic: close of closed channel

The new test TestJobResultFinishIsIdempotent in core/types/result_finish_test.go reproduces this; it panics on main and passes with this change.

Fix

Finish is now idempotent. Under the job lock it checks whether ready is already closed and returns early if so. The first call wins: its error is stored, the channel is closed once, and the finalizers run exactly once. Later calls are no-ops. WaitResult and all other callers are unaffected.

Testing

• go test ./core/types/... — fails (panic) without the fix, passes with it
• go vet ./core/types/ — clean

🤖 Generated with Claude Code

https://claude.ai/code/session_01PHcZ8CTpZEoK3qgroNnkB4

[mudler-agent]

Merged into main in f2a2af4, together with the other PRs in this batch (branch merge/pr-batch-20260927). Thanks!