feat(ctl): add kmeshctl namespace list command for mesh discovery

[devGPP23]

Problem

While looking into the requirements for the upcoming MCP server implementation (Issue #1800), I noticed that we needed a way to retrieve all mesh namespaces (specifically for Tool #10: get_mesh_namespaces). However, there was no native CLI command available to list the namespaces that are actively enrolled in Kmesh, forcing users to rely on manual kubectl queries.

If a user tried to query namespaces via the CLI, it just failed:

$ kmeshctl namespace list
Error: unknown command "namespace" for "kmeshctl"
Run 'kmeshctl --help' for usage.

$ kubectl get namespace -l istio.io/dataplane-mode=Kmesh
NAME            STATUS   AGE
default         Active   10d
bookinfo        Active   2d

Solution
I added a new kmeshctl namespace list subcommand to solve this.

Here is exactly what I did:

• Created the namespace subcommand which queries the Kubernetes API directly.
• Filtered the results server-side using the istio.io/dataplane-mode=Kmesh label so it only returns namespaces that are actually enrolled in the mesh.
• Formatted the output into a clean tabwriter table that displays the namespace name and explicitly shows if there is a waypoint attached by checking the istio.io/use-waypoint label.
• Added unit tests for the command to ensure it's wired up correctly and to meet the >80% test coverage requirements for the MCP tools.

This PR lays the exact groundwork needed so that the MCP server can directly wrap this CLI command to discover namespaces!

User-Facing Changes
Users can now easily discover which namespaces are enrolled in Kmesh and see their waypoint status at a glance without having to run complex kubectl get ns -l ... queries.

Terminal Output:

When there are Kmesh-enrolled namespaces in the cluster:

$ kmeshctl namespace list
NAME            WAYPOINT
default         None
bookinfo        waypoint
kmesh-system    None

[kmesh-bot]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 39.58%. Comparing base (c6357aa) to head (748c39e).
⚠️ Report is 14 commits behind head on main.
see 2 files with indirect coverage changes

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[LiZhenCheng9527]

Is this PR still a draft?

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[devGPP23]

hi @LiZhenCheng9527
the implementation is actually complete & It is no longer a draft on my end

The reason it's still marked as a Draft is that I am currently blocked from clicking the 'Ready for Review' button because I've hit the open pull request limit for the repository (this is my 6th open PR).

I have 2 other open PRs that already have the lgtm label. Could you please merge those? Merging the approved ones will free up my limit and allow me to mark this PR as ready. Thanks

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