Fix data mapping function detection to exclude regular and natural expression functions in data_mappings.bal

[dulajdilshan]

Purpose

Addresses wso2/product-ballerina-integrator#26

Fixed the issue where all functions in data_mappings.bal were incorrectly marked as data mapping functions. The fix ensures that only expression-bodied functions (excluding natural expression functions) are marked as data mapping functions.

All Cases Covered:

1. ✅ Expression-bodied functions in data_mappings.bal (like transform, mapPersonToEmployee) → isDataMappedFunction: true
2. ✅ Regular functions with block body in data_mappings.bal (like customHelper) → isDataMappedFunction: false
3. ✅ Isolated functions in data_mappings.bal (like validateAge) → isDataMappedFunction: false
4. ✅ Natural expression functions in data_mappings.bal (like generateGreeting) → Skipped entirely (not shown in results)
5. ✅ Natural expression functions in functions.bal → Skipped entirely (existing behavior maintained)

Approach

Reference: wso2/product-ballerina-integrator#26 (comment)

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[pasindufernando1]

LGTM