fix: prevent loss of step log output at end of step (#1028)

## Problem

Several runner code paths could drop the **tail** of a step's log output, so a
failing (or cancelled) step would show output that is missing its last line(s).
This was observed in practice and traced to four independent issues.

## Root causes & fixes

### 1. Trailing line without a newline was never flushed
`common.lineWriter` buffers output until it sees a `\n`. A final line **without**
a trailing newline (e.g. an error message printed right before a process exits,
a panic, `printf` without `\n`) stayed in the internal buffer and was never
emitted — the writer exposed no flush at all.

- Added `lineWriter.Flush()` (idempotent), a `Flusher` interface, and a
  `FlushWriter(io.Writer)` helper.
- Flush at every stream EOF: the exec copy goroutine, the container `attach()`
  streaming goroutine, and at step end (`useStepLogger`).

### 2. Cancellation/timeout truncated output
`waitForCommand` returned immediately on `ctx.Done()` and abandoned the
output-copy goroutine, losing output the command had already produced. It now
drains with a bounded grace period before returning. The response channel is
buffered so the goroutine can't leak if the drain times out.

### 3. `attach()` raced the final bytes
Container output was streamed in a fire-and-forget goroutine that `wait()` did
not synchronize with, so the step could proceed before the last bytes were
written. `wait()` now blocks on the streaming goroutine (bounded) so output is
fully drained and flushed first.

### 4. `::stop-commands::` silently dropped lines from the step log
Lines between `::stop-commands::<token>` and its end token were echoed without
the `raw_output` field **and** short-circuited the handler chain (`return false`),
so they never reached the step log (non-raw entries aren't appended while a step
is running). Now returns `true` so they are still captured.

Reviewed-on: https://gitea.com/gitea/runner/pulls/1028
Reviewed-by: Zettat123 <39446+zettat123@noreply.gitea.com>

act/runner/command.go

@@ -48,8 +48,11 @@ func (rc *RunContext) commandHandler(ctx context.Context) common.LineHandler {
 		if resumeCommand != "" && command != resumeCommand {
 			// There should not be any emojis in the log output for Gitea.
 			// The code in the switch statement is the same.
+			// Return true (not false) so the line still reaches the raw_output
+			// log handler; otherwise everything between ::stop-commands:: and
+			// its end token is silently dropped from the step log.
 			logger.Infof("%s", line)
-			return false
+			return true
 		}
 		arg = UnescapeCommandData(arg)
 		kvPairs = unescapeKvPairs(kvPairs)