DAOS-18304 ddb: Add Go unit tests for ddb command layer

[knard38]

Description

Overview

Go unit test coverage for the ddb command layer, built on top of the DdbAPI
interface introduced in the predecessor PR #17967.

Tests use a DdbContextStub that implements DdbAPI, so they run without a
live VOS environment and without CGo compilation.

New Files

test_helpers.go

Test infrastructure shared across all test files:

• DdbContextStub: a pure-Go implementation of DdbAPI that records calls
  and returns configurable errors, used as a drop-in replacement for DdbContext.
• captureStdout: redirects os.Stdout to a pipe for the duration of a
  function call and returns the captured output.
• runCmdToStdout: calls parseOpts() with given args and captures stdout.
• runMainFlow: simulates the full main() execution flow (parse → version
  check → run) without calling os.Exit().
• isArgEqual: type-safe argument comparison helper for stub assertions.

ddb_commands_test.go

Tests for the grumble command definitions:

• TestHelpCmds: verifies --help output for the ls and open commands.
• TestCmds: argument and option parsing for ls (path, --recursive,
  --details) and open (--vos_path, --db_path, --write_mode).
• TestManPage: man page output to stdout and to file.

main_test.go

Tests for the CLI entry-point logic:

• TestParseOpts: general help, unknown commands with --help, default
  option values, flag parsing (--debug, --write, --vos_path, --db_path,
  --version) and error cases (conflicting --cmd / --cmd_file, missing
  --vos_path when --db_path is set).
• TestRun: version output, unknown command detection, auto-open behaviour
  with --vos_path and --db_path, command-line and command-file execution modes.
• TestStrToLogLevels: all supported log level string conversions.

Steps for the author:

• Commit message follows the guidelines.
• Appropriate Features or Test-tag pragmas were used.
• Appropriate Functional Test Stages were run.
• At least two positive code reviews including at least one code owner from each category referenced in the PR.
• Testing is complete. If necessary, forced-landing label added and a reason added in a comment.

After all prior steps are complete:

• Gatekeeper requested (daos-gatekeeper added as a reviewer).

[github-actions]

Ticket title is 'Add unit test to ddb go code'
Status is 'In Progress'
https://daosio.atlassian.net/browse/DAOS-18304