Mux recovery when link up comes before ICMP session Active with ICMP Offload

[harjotsinghpawra]

Description of PR

Solving the bug where is icmp hwoffload reports icmp session up before Link Up . When prober goes active first and MUX is Unknown state then Prober is in Wait but mux gets stuck and Mux never recovers, even after Link comes back up.

The transition handler for (Active, Unknown, Up) is LinkProberActiveMuxUnknownLinkUpTransitionFunction. Because ps(mCompositeState) is already Active when the link-up arrives, it takes the else branch and calls startMuxProbeTimer(), which:

• Calls probeForwardingState() to ask xcvrd for the current forwarding state.
• Arms mDeadlineTimer; if xcvrd does not respond, it retries with backoff.
• When xcvrd replies Active, handleProbeMuxStateNotification(Active) drives the final switchMuxState(Active) to orchagent.

Scenario	Before fix	After fix
ICMP session UP before link UP (HW offload)	(P: Active, M: Unknown, L: Down) → link up → (P: Wait, M: Unknown, L: Up) — stuck, no recovery	(P: Active, M: Unknown, L: Down) → link up → (P: Active, M: Unknown, L: Up) → xcvrd probe → (P: Active, M: Active, L: Up) — recovered
SW prober or link UP before ICMP session	initLinkProberState() runs as before — no change	Same, initLinkProberState() runs as before — no change
Prober already Active, mux already Active, link recovers	initLinkProberState() resets prober unnecessarily	Transition handler sees (Active, Active, Up) — no redundant switchMuxState issued

Issue raised:
sonic-net/sonic-buildimage#26547

Summary:
Fixes # (issue)

Type of change

• Bug fix
• New feature
• Doc/Design
• Unit test

Approach

What is the motivation for this PR?

sonic-net/sonic-buildimage#26547
Bug reported

Work item tracking

• Microsoft ADO (number only):

How did you do it?

Forced it to do get grpc probe for MUX state if link goes from Up from down and prober is in active
then it will do state transition and MUX will resolve itself using grpc.

How did you verify/test it?

Unit test included in patch

Any platform specific information?

Documentation

[mssonicbld]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[harjotsinghpawra]

@zjswhhh Please review it if possible.