fix: improve behaviour across masking, commands and status (#1194)

Fixes 51 bugs discovered via comparison with `actions/runner`. Every fix has test coverage.

### Secrets

- A short secret registered no shifted-base64 form, so `base64("user:$TOKEN")` printed in the clear
- Encoded forms came only from the whole trimmed value, missing padded and per-line spellings
- Masks split only on `\n`, so `::add-mask::a%0Db` registered neither half
- Adds XML, expression-string and quote-trimming encoders

### Workflow commands

- Split at the last `::` or `]` rather than the first, so `::add-mask::a::b` registered no mask
- A command on the last line without a newline was ignored, and `::ADD-MASK::` did nothing
- `##[...]` did not decode `%3B`/`%5D`, properties lost anything after a second `=`
- `$GITHUB_ENV` and `::set-env::` now refuse `NODE_OPTIONS`

### Status

- `continue-on-error` reported failed, a cancelled job reported success, an `if:` error reported cancelled
- File commands ran after `continue-on-error`, failing the job while the step stayed green
- A bad job output aborted the whole run instead of that job

### Steps and actions

- `${{ matrix.* }}` and `${{ strategy.* }}` were empty inside composite actions
- Composite inputs leaked into nested actions as `INPUT_*`, `with:` matched case-sensitively, `pre` failures were dropped
- Docker actions dropped `runs.env` when the caller passed `with: args:`, and caller `args`/`entrypoint` beat the manifest
- An implicit shell ran with `pipefail`, a `shell:` without `{0}` passed without running
- `container.env` overrode job env and every `$GITHUB_ENV` write, heredocs lost leading blank lines, `$GITHUB_PATH` was not BOM-decoded

Written by Claude Opus 5.

Reviewed-on: https://gitea.com/gitea/runner/pulls/1194
Reviewed-by: bircni <bircni@icloud.com>
Co-authored-by: silverwind <me@silverwind.io>

act/container/parse_env_file.go

@@ -50,7 +50,7 @@ func parseEnvFile(e Container, srcPath string, env *map[string]string) common.Ex
 			case singleLineEnv != -1 && (multiLineEnv == -1 || singleLineEnv < multiLineEnv):
 				localEnv[line[:singleLineEnv]] = line[singleLineEnv+1:]
 			case multiLineEnv != -1:
-				multiLineEnvContent := ""
+				var multiLineEnvContent []string
 				multiLineEnvDelimiter := line[multiLineEnv+2:]
 				delimiterFound := false
 				for s.Scan() {
@@ -59,10 +59,7 @@ func parseEnvFile(e Container, srcPath string, env *map[string]string) common.Ex
 						delimiterFound = true
 						break
 					}
-					if multiLineEnvContent != "" {
-						multiLineEnvContent += "\n"
-					}
-					multiLineEnvContent += content
+					multiLineEnvContent = append(multiLineEnvContent, content)
 				}
 				if err := s.Err(); err != nil {
 					return fmt.Errorf("reading env file: %w", err)
@@ -70,7 +67,7 @@ func parseEnvFile(e Container, srcPath string, env *map[string]string) common.Ex
 				if !delimiterFound {
 					return fmt.Errorf("invalid format delimiter '%v' not found before end of file", multiLineEnvDelimiter)
 				}
-				localEnv[line[:multiLineEnv]] = multiLineEnvContent
+				localEnv[line[:multiLineEnv]] = strings.Join(multiLineEnvContent, "\n")
 			default:
 				return fmt.Errorf("invalid format '%v', expected a line with '=' or '<<'", line)
 			}