feat(file-logger): use io.stdout/io.stderr for /dev/stdout and /dev/stderr paths

[ecsimsw]

Description

• The file-logger plugin uses io.open(path, "a+"), even when path is /dev/stdout or /dev/stderr
• This depends on OS file permissions, which fails with Permission denied in restricted environments
• This PR uses io.stdout or io.stderr Lua runtime handles, which are always available regardless of file permissions
• No schema changes, existing "path": "/dev/stdout" configuration works as before

Changes

• apisix/plugins/file-logger.lua : use io.stdout/io.stderr handles instead of io.open when path is /dev/stdout or /dev/stderr
• t/cli/test_file_logger_stdout.sh : add CLI test for stdout output
• docs/en/latest/plugins/file-logger.md : document /dev/stdout and /dev/stderr in path attribute
• docs/zh/latest/plugins/file-logger.md : same for Chinese docs

Note on test approach

• The CLI test runs openresty directly in the foreground instead of using apisix start.
• Because apisix start launches nginx via io.popen(cmd):read("*all") which consumes the process stdout, making /dev/stdout output unobservable from outside.
• This is the same approach used in production containers (docker-entrypoint.sh).

Which issue(s) this PR fixes:

Fixes #6797

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible