Decouple CLI optionality from runtime JetBrains annotation fork

[bbottema]

Context

Issue #610 reports a Dependency-Check finding for com.github.bbottema:jetbrains-runtime-annotations:1.0.2 being mapped to JetBrains Runtime / CVE-2025-29903. The artifact used here is not JetBrains Runtime; it is a republished copy of JetBrains @NotNull / @Nullable with RUNTIME retention so the CLI generator can inspect @Nullable through reflection.

That design made the CLI clever but brittle: Java/API nullability and CLI argument optionality are currently coupled through the same annotation.

Goal

Separate those meanings explicitly:

• Keep official org.jetbrains:annotations for API nullability and standard IDE/static-analysis tooling.
• Remove com.github.bbottema:jetbrains-runtime-annotations from Simple Java Mail.
• Add explicit runtime-retained CLI metadata, probably @Cli.Optional or @Cli.OptionalParameter, for builder parameters that are optional in the CLI.
• Update CLI generation and command-line consumption to inspect that CLI annotation instead of org.jetbrains.annotations.Nullable.
• Preserve Java 8 compatibility.
• Document the builder/Javadoc/reflection/serialized-metadata CLI mechanism in PROJECT_MECHANISMS_CATALOGUE.md, including why the organization is brittle and how future API changes should keep CLI parity.

Notes

This should address the practical scanner/tooling concern from #610 without renaming the runtime fork or continuing to depend on the fork in Simple Java Mail.

[bbottema]

Implemented by decoupling CLI optionality from Java nullability:

• Removed the com.github.bbottema:jetbrains-runtime-annotations dependency from Simple Java Mail.
• Kept official org.jetbrains:annotations for API nullability and tooling compatibility.
• Added runtime-retained @Cli.Optional for CLI-reflected builder parameters.
• Updated CLI option declaration and command-line consumption to use @Cli.Optional instead of reflective @Nullable.
• Added CLI coverage for both option metadata and runtime parsing with omitted/provided optional arguments.
• Documented the CLI/Javadoc/reflection mechanism and API expansion expectations in the project catalogue/workflow docs.

This resolves the scanner/tooling problem raised in #610 without keeping the runtime annotation fork in the dependency graph.

[bbottema]

Released in Simple Java Mail v9.0.0, which is now available from Maven Central:

https://repo1.maven.org/maven2/org/simplejavamail/simple-java-mail/9.0.0/

Release notes:
https://github.com/bbottema/simple-java-mail/releases/tag/9.0.0