Add OnConnect and OnDisconnect callbacks to ManagedConnection

[prashanthjos]

This enables consumers to react to connection state changes without polling. Callbacks are invoked when:

• OnConnect: connection is established (including reconnections)
• OnDisconnect: connection is lost (with error reason)

Changes

• Add OnConnect callback field to ManagedConnection - invoked when connection is established
• Add OnDisconnect callback field to ManagedConnection - invoked when connection is lost with error reason
• Add tests for callback functionality

/kind enhancement

Fixes #

This change supports observability improvements in knative/serving#16318, where the activator needs to track autoscaler connection status. With callbacks, consumers can update metrics and logs in real-time without polling Status().

Release Note

Add OnConnect and OnDisconnect callbacks to websocket ManagedConnection for event-driven connection state monitoring

[knative-prow]

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[knative-prow]

Hi @prashanthjos. Thanks for your PR.

I'm waiting for a knative member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.66%. Comparing base (93d6642) to head (31d06d1).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3309      +/-   ##
==========================================
+ Coverage   74.63%   74.66%   +0.03%     
==========================================
  Files         188      188              
  Lines        8195     8207      +12     
==========================================
+ Hits         6116     6128      +12     
  Misses       1840     1840              
  Partials      239      239              

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[prashanthjos]

@dprotaso can you please have a look at this PR, this is a pre-required PR for

knative/serving#16318 (comment)

[prashanthjos]

The callbacks need to be set before the connection goroutine starts to avoid a data race. When NewDurableSendingConnection returns, the background goroutine is already running and may read OnConnect/OnDisconnect at any time.

If we allowed setting callbacks after construction like:

conn := NewDurableSendingConnection(target, logger)
conn.OnConnect = func() { ... }  // RACE: goroutine may already be reading this

There's a race between the main goroutine writing the callback and the connection goroutine reading it.

The functional options pattern ensures callbacks are applied atomically before the goroutine starts:

conn := NewDurableSendingConnection(target, logger,
    WithOnConnect(func() { ... }),
    WithOnDisconnect(func(err error) { ... }),
)