fix(actions): make SingleWorkflow.Marshal round-trip multi-line run blocks (stop silent job stranding) (#38520)

## Problem
Jobs that call a reusable workflow (`uses:`) whose steps contain a `run:
|` block that **starts with blank lines** never start — the child jobs
stay `Blocked` forever, the run never finishes, and nothing is shown to
the user (the error is only logged at DEBUG). The reusable is valid YAML
and worked on 1.26.

## Root cause
`jobparser` serializes each expanded job via `SingleWorkflow.SetJob()`,
which uses `yaml.NewEncoder(...).SetIndent(2)`, but
`SingleWorkflow.Marshal()` used `yaml.Marshal`, whose default
indentation is **4**. Re-emitting a multi-line literal block scalar at a
different indentation makes the encoder write a wrong explicit
indentation indicator (`run: |4`) whose declared indent doesn't match
the actual content indent. The stored `workflow_payload` is then
unparseable:

```
run: |4


                while ...
```

`jobparser.Parse` / `model.ReadWorkflow` (both go.yaml.in/yaml/v4)
reject it: `did not find expected key`. This surfaces in
`services/actions/job_emitter.go` `resolve()` →
`updateConcurrencyEvaluationForJobWithNeeds` → `ParseJob`, where the
error is swallowed at `log.Debug` and the job is left `Blocked`.

Encoding at indent 4 triggers the bad indicator; indent 2 does not —
matching the value already used by `SetJob`.

## Fix
Encode `SingleWorkflow.Marshal()` with `SetIndent(2)` so both encoders
agree and the serialized single workflow round-trips. Adds a regression
test (`Parse → Marshal → Parse` on a `run:` block with leading blank
lines) that fails before the change with `did not find expected key`.

---------

Signed-off-by: quasimodo <mohamed.belkamel@intelcia.com>
Co-authored-by: quasimodo <mohamed.belkamel@intelcia.com>
Co-authored-by: Zettat123 <zettat123@gmail.com>

modules/actions/jobparser/model.go

@@ -75,7 +75,22 @@ func (w *SingleWorkflow) SetJob(id string, job *Job) error {
 }
 
 func (w *SingleWorkflow) Marshal() ([]byte, error) {
-	return yaml.Marshal(w)
+	// Encode with the same indentation SetJob uses (2). yaml.Marshal's default
+	// indentation (4) makes the encoder emit multi-line block scalars (e.g. a
+	// `run:` step that begins with blank lines) with a wrong explicit indentation
+	// indicator (`run: |4`) that then fails to re-parse, which silently strands
+	// the job during concurrency evaluation. Keeping both encoders at indent 2
+	// makes the serialized single workflow round-trip.
+	var buf bytes.Buffer
+	enc := yaml.NewEncoder(&buf)
+	enc.SetIndent(2)
+	if err := enc.Encode(w); err != nil {
+		return nil, err
+	}
+	if err := enc.Close(); err != nil {
+		return nil, err
+	}
+	return buf.Bytes(), nil
 }
 
 type Job struct {