feat: finalize TriggerV2 cross-namespace BrokerRef support

[jijo-OO7]

TriggerV2: Cross-namespace BrokerRef Support | Fixes #4080

This PR continues the work started in #4060 to add support for
cross-namespace BrokerRef for TriggerV2 in the Kafka Broker.

The core implementation enabling cross-namespace Trigger reconciliation
was authored by @Cali0707 and is reused here without functional changes.

What this PR does

This PR completes the feature by adding the missing validation and coverage:

• Adds unit tests for TriggerV2 cross-namespace BrokerRef behavior
• Adds broker-based E2E tests validating cross-namespace Trigger usage
• Ensures Trigger reconciliation succeeds when Triggers and Brokers
  exist in different namespaces and the cross-namespace-event-links
  feature is enabled

Context

The functional implementation already exists; this PR focuses on making the
feature mergeable and production-ready by providing the required test
coverage.

[knative-prow]

Hi @jijo-OO7. Thanks for your PR.

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[knative-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: jijo-OO7
Once this PR has been reviewed and has the lgtm label, please assign matzew for approval. For more information see the Code Review Process.

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Details

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[codecov]

Codecov Report

❌ Patch coverage is 38.88889% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 36.12%. Comparing base (f9ffd24) to head (efa9b47).
⚠️ Report is 19 commits behind head on main.

Files with missing lines	Patch %	Lines
...ol-plane/pkg/reconciler/trigger/v2/controllerv2.go	22.22%	7 Missing ⚠️
...ntrol-plane/pkg/reconciler/trigger/v2/triggerv2.go	55.55%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #4621      +/-   ##
============================================
+ Coverage     28.77%   36.12%   +7.34%     
- Complexity        0      901     +901     
============================================
  Files           294      403     +109     
  Lines         16169    20011    +3842     
  Branches          0      342     +342     
============================================
+ Hits           4653     7228    +2575     
- Misses        11063    12144    +1081     
- Partials        453      639     +186     

Flag	Coverage Δ
java-unittests	67.15% <ø> (?)

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