fix(ctl): preserve waypoint labels with revision

[sivakumar-mahalingam]

What type of PR is this?

/kind bug

What this PR does / why we need it:

Preserves existing Gateway labels when adding istio.io/rev, so using --for and --revision together retains both labels. Adds a focused regression test covering both flags, each flag separately, and neither flag.

Which issue(s) this PR fixes:

Fixes #1952

Does this PR introduce a user-facing change?:

Fix `kmeshctl waypoint` generation to preserve the `istio.io/waypoint-for` label when `--revision` is also specified.

Tests:

• go test -count=1 ./ctl/waypoint
• go vet ./ctl/waypoint

[kmesh-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[kmesh-bot]

Welcome @sivakumar-mahalingam! It looks like this is your first PR to kmesh-net/kmesh 🎉

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 39.62%. Comparing base (09e96d3) to head (5eb7809).
⚠️ Report is 3 commits behind head on main.
see 2 files with indirect coverage changes

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[sivakumar-mahalingam]

The two Istio 1.28 E2E jobs were cancelled at the workflow’s 40-minute timeout. DCO, builds, and the other E2E jobs passed. I don’t have permission to rerun the cancelled jobs. @nlgwcy @LiZhenCheng9527 Could you please rerun them?