Fix/3788 always register profile workspace cli

[ugbotueferhire]

[Fix] – Always register dlt profile and dlt workspace CLI commands

Closes #3788

Description

Context:
The dlt profile and dlt workspace CLI commands were completely invisible when no .dlt/.workspace marker file existed. The plugin hooks plug_cli_profile() and plug_cli_workspace() in dlt._workspace.cli._plugins returned None when is_workspace_active() was False. Since pluggy silently filters None from hook results, argparse never learned these commands existed — they were absent from dlt --help and returned "invalid choice" when invoked directly. This broke discoverability for users, AI agents, and documentation references to dlt profile <name> pin.

Approach:
Added lightweight placeholder command classes that are registered unconditionally, following the same graceful-degradation pattern used by DeployCommand.execute() (which prints guidance when pipdeptree/cron_descriptor are missing rather than hiding the command):

• _ProfilePlaceholder added to dlt/_workspace/cli/commands.py — stands in for ProfileCommand when workspace context is inactive. Accepts any arguments via nargs="*" and prints actionable setup instructions on execute.
• _WorkspacePlaceholder added to dlt/_workspace/cli/commands.py — identical pattern for WorkspaceCommand.
• plug_cli_profile() in dlt/_workspace/cli/_plugins.py — returns _ProfilePlaceholder instead of None when workspace is inactive.
• plug_cli_workspace() in dlt/_workspace/cli/_plugins.py — returns _WorkspacePlaceholder instead of None when workspace is inactive.

No changes to the SupportsCliCommand protocol, the argparse wiring in _dlt.py, or any other command. When .dlt/.workspace exists, the real commands are returned exactly as before.

Impact:

• Functionality: dlt profile and dlt workspace are now always visible in dlt --help and print clear setup instructions when invoked outside a workspace context, instead of silently vanishing.
• Developer Experience: Users and AI agents that read docs referencing dlt profile <name> pin now get an actionable error with exact remediation steps (mkdir -p .dlt && touch .dlt/.workspace) instead of a cryptic argparse rejection.
• Robustness: Zero regression risk — when workspace is active, the original ProfileCommand and WorkspaceCommand are still returned by the plugin hooks. The placeholder classes are only used in the fallback path.

Visual Proof

Tests

• New tests added
• Existing tests unaffected (no regressions)

Manual verification performed:

• dlt --help lists both profile and workspace without .dlt/.workspace ✅
• dlt profile list prints setup instructions and exits non-zero ✅
• dlt workspace info prints setup instructions and exits non-zero ✅
• With .dlt/.workspace present, real ProfileCommand / WorkspaceCommand execute normally ✅

[ugbotueferhire]

Hi @anuunchin , whenever you have a moment, could you please take a look at this PR for a review? Let me know if you need any changes. Thanks!

[ugbotueferhire]

Hey @anuunchin, following up on this PR. Pretty low-risk change, would appreciate a review when you have bandwidth or if you could assign someone to this, thanks :)