XDB v1.3.0 - implement atomic update and upsert

[1.3.0] - 2026-01-30

Added

• Atomic Update: Implemented db_update function for efficient document updates without altering the _id.
• Upsert Logic: Introduced db_upsert function to automatically determine whether to perform an insert or update based on ID existence.
• Protocol Extension: Integrated update and upsert actions into server.c for network-layer access.
• Regression Tests: Added new test cases in test_crud.c to verify data update workflows.

Fixed

• Compiler Warnings: Added <stdlib.h> header in unit tests to resolve implicit free() declaration issues.
• Test Compatibility: Replaced ASSERT_STR_EQ macro with standard strcmp in test_crud.c to ensure compilation stability.