Improve selectivity estimation for CASE and SubPlan IS NULL patterns

[RuchaSK1]

Description

Implementing planner hooks that provide accurate selectivity for:

• OpExpr: Decomposes CASE expressions compared to constants using
  conditional probability across branches
• NullTest: Uses SubPlan estimated row count to determine IS NULL
  probability instead of the default 99.5%

These hooks fix row count misestimates in SSMS Object Explorer queries that use the is_ms_shipped CASE filter pattern, improving plan choices from Nested Loop to Hash Join.

Issues Resolved

[List any issues this PR will resolve]

Test Scenarios Covered

• Use case based -

• Boundary conditions -

• Arbitrary inputs -

• Negative test cases -

• Minor version upgrade tests -

• Major version upgrade tests -

• Performance tests -

• Tooling impact -

• Client tests -

Check List

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