BAEL-9185: Introduction to Solon

[jsfan3]

Adds the approved solon-introduction module for BAEL-9185. The example starts with a greeting endpoint and builds a task REST API with Solon 4.1.0, MyBatis-Flex 1.11.8, HikariCP 7.1.0, and H2 2.5.250.

The module targets Java 21 and participates in the repository's default and integration profiles. It inherits com.baeldung:parent-modules and imports solon-parent as a BOM. Unit/integration test selection uses the shared parent's standard filters; there is no custom test.includes property. Compiler parameter metadata is retained for HTTP binding. JUnit 5.14.4, the test-scoped Platform launcher 1.14.4, and Surefire 3.5.5 are aligned to avoid test discovery failures.

Tests cover service validation, the CRUD lifecycle over real HTTP, invalid input, missing resources, and transaction rollback verified before cleanup. Integration tests allocate an available port and use a separate in-memory database. The public integration test class and public rollback method are required by Solon's test extension and proxy interception.

Validation with Temurin 21.0.12.1+1 and Maven 3.9.16:

• mvn -B -ntp -f solon-introduction/pom.xml -Pdefault clean install: 7 unit tests passed
• mvn -B -ntp -pl solon-introduction -Pintegration clean install: 5 integration tests passed, including actual HTTP startup and requests
• Inherited PMD checks passed in both builds
• git diff --check: passed

Java application/test sources required no changes for this build alignment. The redundant usage README was removed; the article explains execution and testing. The diagram's DOT source and regeneration instructions remain in diagrams/info.txt, preserving the author's subsequent cleanup commits.