fix: Close response body when watch content-type negotiation fails

[karlkfi]

What type of PR is this?

/kind bug
/sig api-machinery

What this PR does / why we need it:

When Request.Watch gets a 200 response but the negotiator has no stream decoder for the response content type, newStreamWatcher returns the error without closing resp.Body, leaking the connection.

The fix closes the body on that error path without draining it: a watch response can be a stream held open by the server, so draining could block indefinitely, and the connection cannot be reused anyway. The test holds the stream open to pin that Watch returns promptly.

Structured as two commits: the first extends TestWatchUnknownContentType with the in-package readTracker close counter and fails without the fix, the second is the fix. Test output without the fix:

=== RUN   TestWatchUnknownContentType
    request_test.go:2271: Expected response body Close to be invoked 1 time, but got: 0
--- FAIL: TestWatchUnknownContentType (0.00s)

With the fix, -race and stress are clean:

$ go test -race ./rest/...             # k8s.io/client-go
ok  	k8s.io/client-go/rest	18.041s
ok  	k8s.io/client-go/rest/watch	1.701s
$ stress -p 8 -count 200 rest.test -test.run 'TestWatch'
1m45s: 200 runs total, 0 failures

Which issue(s) this PR fixes:

Fixes #140464

Special notes for your reviewer:

Follow-up to #131706, which cleaned up response body handling in the watch tests. Originally identified as a TODO in #131505.

/cc @liggitt @jpbetz

Does this PR introduce a user-facing change?

Fixed a response body leak in the watch client when the server returns an unsupported content type.

[kubernetes-prow]

This issue is currently awaiting triage.

If a SIG or subproject determines this is a relevant issue, they will accept it by applying the triage/accepted label and provide further guidance.

The triage/accepted label can be added by org members by writing /triage accepted in a comment.

Details

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[kubernetes-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: karlkfi
Once this PR has been reviewed and has the lgtm label, please assign aojea for approval. For more information see the Code Review Process.

The full list of commands accepted by this bot can be found here.

Details

Needs approval from an approver in each of these files:

• staging/src/k8s.io/client-go/OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment