Standardize error message formatting

[hakanensari]

Summary

Standardizes all error messages to use lowercase for consistency with Ruby conventions and improves type error messages to include attribute names for easier debugging.

Changes

Error Message Standardization:

• "Cannot specify both type and block for :name" → "cannot specify both type and block for :name"
• "Cannot specify X as type" → "cannot specify X as type for :attr"

Implementation:

• Changed error messages in Builder and Types to lowercase
• Added exception handling in Builder#coercions to catch and re-raise ArgumentError with attribute context
• Updated tests to match new error format

Before/After

Before:

Structure.new { attribute(:value, Object.new) }
# ArgumentError: Cannot specify #<Object:0x...> as type

After:

Structure.new { attribute(:value, Object.new) }
# ArgumentError: cannot specify #<Object:0x...> as type for :value

Now developers immediately know which attribute has the invalid type!

Testing

• All 139 tests pass
• Updated test assertions to verify lowercase messages
• Added assertion to verify attribute name is included in type errors

Closes #16

🤖 Generated with Claude Code