docs: add TSDoc to SecurityPlugin, policy engine interfaces, and InMemoryPolicyEngine

[nuthalapativarun]

Please ensure you have read the contribution guide before creating a pull request.

Link to Issue or Description of Change

Adds TSDoc to core/src/plugins/security_plugin.ts, which was missing inline documentation on its public interfaces and class members:

• PolicyCheckResult — documents outcome and reason fields
• ToolCallPolicyContext — documents tool and toolArgs fields
• BasePolicyEngine.evaluate() — adds @param and @returns
• InMemoryPolicyEngine.evaluate() — adds description and @returns
• SecurityPlugin constructor — adds @param params
• SecurityPlugin.beforeToolCallback — adds @param descriptions for tool, toolArgs, toolContext and documents the return value

Testing Plan

Unit Tests:

• All unit tests pass locally.

Manual End-to-End (E2E) Tests:
Documentation-only change; no runtime behavior is altered. Verified with npm run build, npm run lint, and npm test — all pass.

Checklist

• Read CONTRIBUTING.md
• Self-reviewed code
• Commented hard-to-understand areas
• Added tests proving fix/feature works
• Unit tests pass locally
• Manually tested end-to-end
• Dependent changes merged/published

[nuthalapativarun]

Closing in favour of the consolidated PR #413, as requested by @kalenkevich. All changes from this PR are included there.