test: add regression coverage for SinkBinding preserving existing volumes

[pujitha24]

Motivation:
Issue #8261 reported that SinkBinding's mutation of a Deployment
subject (SinkBinding.Do in pkg/apis/sources/v1/sinkbinding_lifecycle.go)
eliminated pre-existing volumes/volumeMounts on the subject as soon as
K_SINK env vars were injected, filed against v1.14.0. A maintainer
later attempted to reproduce this on a newer release with a Deployment
carrying a custom ConfigMap volume and was unable to reproduce it, and
no further activity or linked fix followed.

Reviewing the current code, SinkBinding.Do only ever appends to
Volumes/VolumeMounts/Env (including the trust-bundle and OIDC token
volume injection paths), and the admission webhook applies the change
as a computed JSON patch diff rather than a wholesale object replace,
so there is no remaining code path that would drop unrelated volumes.
This change has no user-visible behavior impact today; the bug does
not currently reproduce.

Approach:
Add a regression test case to TestSinkBindingDo that starts from a
subject with a pre-existing user-defined Volume and a container
VolumeMount referencing it, and asserts both are still present after
Do() runs alongside the injected K_SINK/K_CA_CERTS/K_CE_OVERRIDES env
vars. This locks in the current correct behavior and will catch a
regression if this code path changes in the future.

Validation:
go build ./...
go test ./pkg/apis/sources/v1/... -run TestSinkBindingDo -v
(new subtest "preserves_existing_volumes_and_volumeMounts_on_subject"
passes, along with all sibling subtests)

/kind bug

NONE

Fixes #8261

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

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Welcome @pujitha24! It looks like this is your first PR to knative/eventing 🎉

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