Implement TestMetadata_Subscriptions E2E test

[Kunal1522]

Fixes #3202

What's in the test

Main test function:

• TestMetadata_Subscriptions - The actual test. Sets up a Broker, deploys two functions (subscriber and producer), fires an event, and checks the subscriber's logs to confirm it received the event through the Broker.
  Helper functions:
• createBroker(t, namespace, name) - Creates a Knative Broker using kubectl apply. Waits for it to be ready before returning.
• deleteBroker(t, namespace, name) - Cleanup helper that deletes the Broker when the test finishes.
• waitForTrigger(t, namespace, functionName) - Waits for the Knative Trigger (created by func subscribe) to reach Ready state. Without this, events might get lost if we send them too early.
  Unused legacy helpers (kept for potential future use):
• sendEventToBrokerWithRetry - Retry wrapper for sending events (not currently used, test uses direct HTTP instead)
• sendEventToBrokerInternal - In-cluster event sending using Kubernetes port-forwarding (not currently used, test deploys a Producer function instead)

[knative-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: Kunal1522
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[knative-prow]

Hi @Kunal1522. Thanks for your PR.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 53.42%. Comparing base (039b926) to head (471e485).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3324      +/-   ##
==========================================
- Coverage   55.09%   53.42%   -1.68%     
==========================================
  Files         170      170              
  Lines       19850    19850              
==========================================
- Hits        10937    10605     -332     
- Misses       7845     8230     +385     
+ Partials     1068     1015      -53     

Flag	Coverage Δ
e2e	40.03% <ø> (+0.44%)	⬆️
e2e go	35.62% <ø> (ø)
e2e node	31.13% <ø> (ø)
e2e python	35.27% <ø> (-0.09%)	⬇️
e2e quarkus	31.28% <ø> (ø)
e2e rust	30.73% <ø> (ø)
e2e springboot	30.75% <ø> (-0.03%)	⬇️
e2e typescript	31.25% <ø> (ø)
integration	?
unit macos-14	44.96% <ø> (-0.01%)	⬇️
unit macos-latest	44.96% <ø> (ø)
unit ubuntu-24.04-arm	45.07% <ø> (ø)
unit ubuntu-latest	45.86% <ø> (ø)
unit windows-latest	44.98% <ø> (ø)

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