Avoid eager MQTT response buffer access

[MrAlias]

Motivation

• A previous change eagerly dereferenced the response buffer via rpkt.UnsafeView() at function entry which can panic if rpkt is nil even when the request buffer (pkt) contains a valid MQTT packet.
• The goal is to make ProcessPossibleMQTTEvent robust for callers that may pass a nil response buffer while preserving existing behavior when the response buffer is present.

Description

• In pkg/ebpf/common/mqtt_detect_transform.go defer the response-buffer dereference and replace the eager rpkt.UnsafeView() with a lazy call inside the error path: ProcessMQTTEvent(rpkt.UnsafeView()).
• In pkg/ebpf/common/mqtt_detect_transform_test.go add a regression test case PUBLISH in request buffer without response buffer and adjust the test harness to pass a nil *largebuf.LargeBuffer when response is absent so the function is exercised with a nil response buffer.
• Applied formatting (gofmt) to the modified files.

Testing

• go test ./pkg/internal/ebpf/mqttparser
• go test ./pkg/internal/largebuf
• go test ./pkg/ebpf/common -run TestProcessPossibleMQTTEvent

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 68.88%. Comparing base (914ffa4) to head (20b3775).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/ebpf/common/mqtt_detect_transform.go	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2515      +/-   ##
==========================================
- Coverage   69.24%   68.88%   -0.36%     
==========================================
  Files         345      345              
  Lines       46747    46795      +48     
==========================================
- Hits        32372    32237     -135     
- Misses      12330    12521     +191     
+ Partials     2045     2037       -8     

Flag	Coverage Δ
integration-test	50.65% <33.33%> (-0.28%)	⬇️
integration-test-arm	26.44% <25.00%> (-1.86%)	⬇️
integration-test-vm-5.15-lts	?
integration-test-vm-6.18-lts	?
k8s-integration-test	35.76% <33.33%> (+0.06%)	⬆️
oats-test	35.34% <33.33%> (-0.02%)	⬇️
unittests	63.36% <100.00%> (+0.05%)	⬆️

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[grcevski]

LGTM!