Mark DSL functions for types as public (#472)

<!-- Provide a brief summary of your changes -->

## Motivation and Context
<!-- Why is this change needed? What problem does it solve? -->
The dsl builders in [this
commit](74c93745cd (diff-598edaa01d))
were mistakenly marked internal and are therefore unusable. I have
simply marked them as public.

## How Has This Been Tested?
<!-- Have you tested this in a real application? Which scenarios were
tested? -->

## Breaking Changes
No

## Types of changes
<!-- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update

## Checklist
<!-- Go over all the following points, and put an `x` in all the boxes
that apply. -->
- [ ] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [ ] My code follows the repository's style guidelines
- [x] New and existing tests pass locally
- [ ] I have added appropriate error handling
- [ ] I have added or updated documentation as needed

## Additional context
<!-- Add any other context, implementation notes, or design decisions
-->

---------

Co-authored-by: Konstantin Pavlov <1517853+kpavlov@users.noreply.github.com>

AGENTS.md

@@ -46,6 +46,12 @@ MCP Kotlin SDK — Kotlin Multiplatform implementation of the Model Context Prot
 - All tests for each module are located in `src/commonTest/kotlin/io/modelcontextprotocol/kotlin/sdk/`
 - Platform-specific tests go in `src/jvmTest/`, `src/jsTest/`, etc.
 - Use Kotest assertions (`shouldBe`, `shouldContain`, etc.) for readable test failures.
+- For nullable objects with nested properties, prefer `shouldNotBeNull { ... }` blocks:
+  ```kotlin
+  content.annotations shouldNotBeNull {
+      priority shouldBe 1.0
+  }
+  ```
 - Use `shouldMatchJson` from Kotest for JSON validation.
 - Mock Ktor HTTP clients using `MockEngine` from `io.ktor:ktor-client-mock`.
 - Always add tests for new features or bug fixes, even if not explicitly requested.