finished PipelineRun never frees its concurrency slot when provider detection fails

📝 Description

When a repository has a concurrency_limit, each running PipelineRun holds
one slot, and the slot is given back when the run finishes. The handover
happens at the end of reportFinalStatus, which promotes the next queued run.

ReconcileKind never gets there when provider detection fails. At
pkg/reconciler/reconciler.go:267 a failure to detect the git provider logs a
message and returns nil. Returning nil tells the work queue the item is
done, so nothing retries, reportFinalStatus never runs, and the finished
run's slot stays occupied forever.

Provider detection is not a pure local check. For GitHub App installs it calls
InitAppClient, which talks to the network and can fail for a moment. If that
moment coincides with a queued run finishing, the repository permanently loses
one slot of capacity. With concurrency_limit: 1 the queue stops moving
entirely until someone restarts the watcher.

A restart does not even help fully: the finished run still carries the
started state annotation, so the queue rebuild counts it as running again
(see the companion issue about InitQueues).

The neighbouring failure paths in reportFinalStatus (secret fetch, client
setup) all return their error and recover on retry. This one is the odd one
out.

Found during a post-merge review of #2890.

🛠️ Suggested fix

Return the error so the rate-limited work queue retries:

return fmt.Errorf("detect provider: %w", err)

Releasing the slot directly before returning nil is not a good alternative,
because it frees capacity without waking the queue, and nothing else will.

🧪 Testing Strategy

• Unit tests: a done PipelineRun whose provider annotations are missing must make ReconcileKind return an error rather than nil

Start in pkg/reconciler/reconciler.go at ReconcileKind around line 267, then read reportFinalStatus and its neighboring error paths. Run or extend the unit test for a done PipelineRun with missing provider annotations; done means ReconcileKind returns an error so the work queue retries and the concurrency slot can be released.