Add PrimingEventMessage

[devcrocod]

• Add PrimingEventMessage type for SSE stream initialization

Motivation and Context

How Has This Been Tested?

Breaking Changes

Types of changes

• 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

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[github-actions]

	Tests	Passed ☑️	Skipped ⚠️	Failed ❌️	Time ⏱
JUnit Test Report	2122 ran	2056 passed	65 skipped	1 failed	4m 4s 780ms

Test	Result	Time ⏱
JUnit Test Report
io.modelcontextprotocol.kotlin.sdk.server.ServerResourcesNotificationSubscribeTest
ServerResourcesNotificationSubscribeTest.removeResource should send resource update notification()[jvm]	❌ failure	10s 183ms

[kpavlov]

See #449