fix(gateway): use TCP target group protocol for TLSRoute on passthrough NLB listeners

[pujitha24]

Motivation:
Issue #4556 reported that a Gateway API NLB listener with protocol: TLS
and tls.mode: Passthrough incorrectly built a TLS listener. That part
was already fixed in v3.1.0 by downgrading the listener protocol to TCP
for passthrough mode.

However, follow-up reports on the same issue (from 2026-03-04 and
2026-03-10) show the fix was incomplete: when a TLSRoute is bound to
such a passthrough listener, the controller still infers the target
group protocol as TLS. This produces a real AWS API error when
reconciling the load balancer:

IncompatibleProtocols: The listener and the following target groups
have incompatible protocols

because the (now TCP) listener cannot be attached to a TLS target group.
This is a functional failure, not cosmetic: the Gateway never reaches a
working state and reconciliation errors repeatedly.

Approach:
inferTargetGroupProtocolFromRoute in
pkg/gateway/model/model_build_target_group.go always returned
ProtocolTLS for TLSRouteKind on an NLB, regardless of the actual
listener protocol. It now takes the (already passthrough-downgraded)
listener protocol and returns ProtocolTCP when the listener protocol
is TCP, keeping ProtocolTLS for the tls.mode: Terminate case. Users
who explicitly set targetGroupProps.Protocol are unaffected, since
that explicit-override path is checked before the inference path.

This only changes NLB behavior for TLSRoute; ALB target group protocol
inference (which cannot take this NLB-only branch) is unaffected.

Validation:
go build ./...
go test -count=1 ./pkg/gateway/model/...
Both pass, including a corrected unit test case in
Test_buildTargetGroupProtocol that had been asserting the buggy
(mismatched) behavior.

Fixes #4556

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

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[kubernetes-prow]

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[kubernetes-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: pujitha24
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[Copilot]

Pull request overview

Fixes Gateway API NLB behavior for TLSRoute when a listener is configured as protocol: TLS with tls.mode: Passthrough: while the listener is already downgraded to TCP, the controller now also infers a TCP target group protocol to avoid AWS “IncompatibleProtocols” reconciliation failures.

Changes:

• Pass listener protocol into target group protocol inference so TLSRoute on NLB can infer TCP when the (passthrough-downgraded) listener protocol is TCP.
• Update the unit test to assert the corrected behavior for passthrough TLS listeners.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
pkg/gateway/model/model_build_target_group.go	Adjusts protocol inference for TLSRoute on NLB to align target group protocol with passthrough-downgraded TCP listeners.
pkg/gateway/model/model_build_target_group_test.go	Updates Test_buildTargetGroupProtocol to expect TCP for the passthrough-downgraded listener case.

---

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[pujitha24]

/assign @zac-nixon

This is rebased and conflict-free whenever you get a chance to take a look.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.52%. Comparing base (057c70f) to head (56f6433).
⚠️ Report is 25 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4853      +/-   ##
==========================================
+ Coverage   57.49%   57.52%   +0.03%     
==========================================
  Files         396      396              
  Lines       31621    31675      +54     
==========================================
+ Hits        18179    18220      +41     
- Misses      12384    12397      +13     
  Partials     1058     1058              

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[zac-nixon]

/lgtm
/approved