API: Propagate allowMissing to nested StructProjection

[sachinnn99]

Fixes #16123.

StructProjection.createAllowMissing() does not propagate allowMissing to nested struct sub-projections. When a nested struct's projected schema contains optional fields not present in the data schema, the constructor throws IllegalArgumentException instead of treating the missing field as null.

The bug is on line 126 where the nested StructProjection is created using the 2-arg constructor, which defaults allowMissing=false.

Fix: Pass allowMissing through to the nested StructProjection constructor.

Tests: Added TestStructProjection covering:

• Nested struct with a missing optional field via createAllowMissing (returns null)
• Same setup with create() throws IllegalArgumentException
• 3-level deep nesting to verify recursive propagation
• All fields present still projects values correctly with allowMissing=true

[sachinnn99]

Closing in favor of #15984 which already covers this fix with comprehensive test coverage.