Fix a bug where fields of aliased structs couldn't be accessed

[MidnightDesign]

foo:{bar: string}.bar worked, but foo:MyStruct.bar didn't.

[Copilot]

Pull Request Overview

This PR fixes a bug where field access on aliased struct types was failing. The issue was that direct field access using array syntax didn't work for aliased types (e.g., foo:MyStruct.bar), while inline struct definitions worked correctly (e.g., foo:{bar: string}.bar).

• Updated field access logic to use getFieldType() method instead of direct array access
• Added proper error handling for unknown fields
• Updated test assertion from assertSame to assertEquals for better object comparison
• Added comprehensive test case covering aliased struct field access in complex expressions

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/FieldAccess.php	Fixed field access by using getFieldType() method and added error handling for unknown fields
tests/unit/ExpressionTest.php	Added test case for aliased struct field access and updated assertion method

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