[DASH] Validate ca to pa SAI attributes

[theasianpianist]

What I did
Only set certain CA to PA attributes if they are valid for a given mapping type

Why I did it
Some SAI attributes are only valid for privatelink and some are only valid for non-privatelink. This is enforced by the SAI metadata layer, so add this check in orchagent to prevent crashes.

How I verified it

Details if related

[mssonicbld]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[Copilot]

Pull request overview

This PR reorganizes the DASH vnet orchestration code to conditionally set SAI attributes for outbound CA to PA entries based on routing type (privatelink vs non-privatelink). This prevents crashes that occur when attempting to set attributes that are only valid for specific routing types, as enforced by the SAI metadata layer.

Key changes:

• Refactored attribute setting logic to separate privatelink-specific attributes from non-privatelink-specific attributes
• Improved test error messages to include operation type (SET/DEL)
• Added explicit return values to mock expectations in tests

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
orchagent/dash/dashvnetorch.cpp	Reorganizes CA to PA attribute setting based on routing type; moves encap_type and tunnel_key extraction to variables for conditional use
tests/mock_tests/mock_dash_orch_test.cpp	Changes op variable from auto to std::string and includes operation type in error messages
tests/mock_tests/dashvnetorch_ut.cpp	Adds explicit Return(SAI_STATUS_SUCCESS) to mock expectations for better test clarity

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[prabhataravind]

lgtm. Please verify this on cisco SS as well.