shell: return flag parse errors from every command (#12618)

* shell: return flag parse errors from every command

48 commands returned nil when their flags failed to parse, so a mistyped
flag printed the flag error but the command still counted as a success:
a piped weed shell run exited 0, the master maintenance loop logged no
error, and admin script jobs reported the line as done.

Return the parse error as s3.bucket.create (#12544) and the other shell
commands already do. TestCommandsReturnFlagParseErrors runs every
registered command with an unknown flag and fails for the 48 on master.

Signed-off-by: anant <170962632+antcybersec@users.noreply.github.com>

* shell: don't report -h as a failure

With the parse errors returned, -h on those commands now returns
flag.ErrHelp, so the shell printed "error: flag: help requested" and a
piped run exited 1. The flag package has already printed the usage, so
processEachCmd now treats flag.ErrHelp as success for every command.

Signed-off-by: anant <170962632+antcybersec@users.noreply.github.com>

* shell: keep the -h test beside the flag parse test

TestProcessEachCmdTreatsHelpAsSuccess covers the same change as
TestCommandsReturnFlagParseErrors, so it lives in the same new file and
shell_liner_test.go stays as it was.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Signed-off-by: anant <170962632+antcybersec@users.noreply.github.com>
Co-authored-by: anant <170962632+antcybersec@users.noreply.github.com>
Co-authored-by: Chris Lu <chrislusf@users.noreply.github.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

weed/shell/command_ec_decode.go

@@ -60,7 +60,7 @@ func (c *commandEcDecode) Do(args []string, commandEnv *CommandEnv, writer io.Wr
 	checkMinFreeSpace := decodeCommand.Bool("checkMinFreeSpace", true, "check min free space when selecting the decode target")
 	batchSize := decodeCommand.Int("batchSize", DefaultEcBatchSize, "decode up to this many volumes per topology refresh (0 = one snapshot for all volumes)")
 	if err = decodeCommand.Parse(args); err != nil {
-		return nil
+		return err
 	}
 	if *batchSize < 0 {
 		return fmt.Errorf("-batchSize must be >= 0")