fix(actions): dynamic matrix expansion correctness fixes (#38690)

Follow-up to https://github.com/go-gitea/gitea/pull/36564 (dynamic
matrix) and https://github.com/go-gitea/gitea/pull/36357 (max-parallel),
fixing issues found reviewing the two features together.

- **A placeholder could stall its run forever.** Its payload keeps the
raw matrix but loses its `needs`, so `ParseJob` re-expanded it instead
of reading it back — fatal for `include: ${{ fromJson(needs.*.outputs.*)
}}`.
- **An `if:` reading `matrix.*` skipped the whole job**, with or without
the `${{ }}`. It now reduces to the needs gate, except under
`always()`/`failure()`/`cancelled()`, and each combination is decided on
its own values once the matrix expands.
- **Dependents could be skipped before the combinations ran**, since
inserted siblings are absent from the resolver's job set. The pass now
stops after an insert and defers to the re-emit it schedules.
- **Expansion failures stranded the placeholder.** A retryable one is
returned so the queue retries it; a malformed payload fails the job
instead of requeueing forever.
- **Rerun could rewind a pass-through row** into a raw placeholder
keeping its old terminal status, which nothing expands. Now gated on the
anchor itself.

Plus: `max-parallel` distinguishes an unevaluated `${{ }}` (debug) from
a non-numeric literal (warn — it silently drops the cap).

Co-authored-by: Zettat123 <zettat123@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>

services/actions/max_parallel_test.go

@@ -29,8 +29,8 @@ func TestParseMaxParallel(t *testing.T) {
 		{"-1.5", 0},          // truncates to -1, which means unlimited
 		{"1e3", 256},         // clamped to MaxJobNumPerRun
 		{"nan", 0},           // must not reach the int cast
-		{"${{ vars.n }}", 0}, // expressions are not evaluated yet
-		{"abc", 0},
+		{"${{ vars.n }}", 0}, // expressions are not evaluated yet, logged as such
+		{"abc", 0},           // a plain workflow error, warned about rather than hidden
 	}
 	for _, tt := range tests {
 		assert.Equal(t, tt.want, parseMaxParallel("job", tt.input), "input %q", tt.input)