fix: show flag completions when completing a double-dash prefix

[morozov]

What type of PR is this?

• bug

What this PR does / why we need it:

When the user types cmd subcommand --<TAB> or cmd subcommand --prefix<TAB>, the CLI should show available flags. Previously, this produced no completions or malformed output because of four bugs:

1. checkShellCompleteFlag() disabled completion mode when -- appeared anywhere in arguments, including when -- was the token being completed.
2. DefaultCompleteWithFlags() returned early when lastArg == "--".
3. parseFlags() stripped -- from args during completion, causing lastArg to be empty and triggering command suggestions instead of flag suggestions.
4. parseFlags() dropped unknown or value-less flags from posArgs and returned an error, so partial prefixes like --prefix were lost before DefaultCompleteWithFlags could use them to filter flag suggestions.

Changes:

• help.go: checkShellCompleteFlag now only disables completion if -- appears before the token being completed, not when it IS the token being completed
• help.go: Removed early return in DefaultCompleteWithFlags for lastArg == "--"
• command_parse.go: Preserve -- in posArgs during shell completion mode
• command_parse.go: Preserve unknown flags and flags missing a value in posArgs during shell completion mode, so partial prefixes reach DefaultCompleteWithFlags

Which issue(s) this PR fixes:

Fixes #2315

Testing

All changes are covered by new/updated tests that fail without the code changes and pass with them:

• TestCommand_Run_CustomShellCompleteAcceptsMalformedFlags: Updated to expect --undefined and --number flags preserved in args
• TestCompletionSubcommand: Added tests for --<TAB> and --partial<TAB> scenarios
• TestDefaultCompleteWithFlags: Updated flag-suggestion-double-dash-shows-all-flags to expect flag output
• Test_checkShellCompleteFlag: Added double dash is the token being completed test case

Release Notes

Fixed shell completion to show flag suggestions when completing `--` or partial flag prefixes like `--prefix`