Exclude linkerd-proxy sidecar ports from port discovery

[pujitha24]

Motivation:
When canary.spec.service.portDiscovery is enabled, Flagger scans the
target Deployment/DaemonSet containers and adds any extra container
ports to the generated canary/primary/apex Services, so multi-port
apps stay reachable. The exclusion list used to skip mesh sidecars
only knew about Istio's container names ("istio-proxy", "envoy").
Linkerd's proxy injector uses the container name "linkerd-proxy", so
its internal ports (e.g. 4143 inbound, 4191 admin) were treated as
application ports and added to the generated Services instead of
being skipped. This was reported as a comment on #1345: "portDiscovery
does not work for linkerd" (using portDiscovery on a linkerd-meshed
deployment).

Note this does not address the original report in #1345, which does
not enable portDiscovery at all — dropping ports that only exist on a
pre-existing Service (and are not declared as container ports) is a
separate, broader problem that needs its own design discussion.

Approach:
Add "linkerd-proxy" to the sidecars exclusion map in
pkg/canary/util.go, matching how "istio-proxy" and "envoy" are
already excluded from getPorts().

Validation:
Added TestGetPortsExcludesSidecars to pkg/canary/util_test.go,
asserting that getPorts() drops linkerd-proxy and istio-proxy
container ports while keeping a regular app container's extra port.

go build ./...
go test ./pkg/canary/... -run TestGetPortsExcludesSidecars -v

Both passed. Also ran the full pkg/canary test suite (go test
./pkg/canary/...) with no regressions.

Fixes #1345

Signed-off-by: Pujitha Paladugu 10557236+pujitha24@users.noreply.github.com

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 30.15%. Comparing base (122fe97) to head (cf6cb64).
⚠️ Report is 30 commits behind head on main.
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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1955      +/-   ##
==========================================
+ Coverage   29.78%   30.15%   +0.37%     
==========================================
  Files         288      288              
  Lines       18591    18641      +50     
==========================================
+ Hits         5537     5622      +85     
+ Misses      12325    12282      -43     
- Partials      729      737       +8     

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[aryan9600]

lgtm! 🏅