feat! Add context to server call handlers (#515)

## Motivation and Context

Adds a `ClientConnection` context receiver to server call handlers,
which provides access to the client communication channel. This is
necessary to allow tools to send notifications, logging, elicitations,
etc.

Supersedes https://github.com/modelcontextprotocol/kotlin-sdk/pull/144,
discussed in
https://github.com/modelcontextprotocol/kotlin-sdk/issues/295.

## How Has This Been Tested?

With the added integration test.

## Breaking Changes

It breaks binary compatibility and breaks source compatibility for tool
handlers that already use `this`.

If you care a lot about maintaining compatibility, I could add some
overloads to ease the transition. But the new methods would likely need
to be named differently for resolution.

Future compatibility should be easier to maintain - `Context` is an
interface (with experimental opt-in subclassing), so new fields can be
added without breaking compatibility. Different types could also be
introduced for different handles without breaking compatibility, as long
as they extend `ClientConnection`.

## Types of changes
<!-- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [ ] Bug fix (non-breaking change which fixes an issue)
- [x] New feature (non-breaking change which adds functionality)
- [x] 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. -->
- [x] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [x] My code follows the repository's style guidelines
- [x] New and existing tests pass locally
- [x] I have added appropriate error handling
- [x] 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>

kotlin-sdk-server/detekt-baseline-commonMainSourceSet.xml

@@ -5,17 +5,9 @@
     <ID>MagicNumber:StdioServerTransport.kt:StdioServerTransport$8192</ID>
     <ID>MaxLineLength:SSEServerTransport.kt:SseServerTransport$"SSEServerTransport already started! If using Server class, note that connect() calls start() automatically."</ID>
     <ID>MaxLineLength:SSEServerTransport.kt:SseServerTransport$*</ID>
-    <ID>MaxLineLength:ServerSession.kt:ServerSession$"Client requested unsupported protocol version $requestedVersion, falling back to $LATEST_PROTOCOL_VERSION"</ID>
-    <ID>MaxLineLength:ServerSession.kt:ServerSession$"Creating message with ${params.params.messages.size} messages, maxTokens=${params.params.maxTokens}, temperature=${params.params.temperature}, systemPrompt=${if (params.params.systemPrompt != null) "present" else "absent"}"</ID>
     <ID>ReturnCount:KtorServer.kt:private suspend fun existingStreamableTransport: StreamableHttpServerTransport?</ID>
-    <ID>ThrowsCount:ServerSession.kt:ServerSession$override fun assertCapabilityForMethod</ID>
-    <ID>ThrowsCount:ServerSession.kt:ServerSession$override fun assertNotificationCapability</ID>
-    <ID>ThrowsCount:ServerSession.kt:ServerSession$override fun assertRequestHandlerCapability</ID>
     <ID>TooGenericExceptionCaught:SSEServerTransport.kt:SseServerTransport$e: Exception</ID>
-    <ID>TooGenericExceptionCaught:Server.kt:Server$e: Exception</ID>
     <ID>TooGenericExceptionCaught:StdioServerTransport.kt:StdioServerTransport$e: Throwable</ID>
     <ID>TooManyFunctions:KtorServer.kt:io.modelcontextprotocol.kotlin.sdk.server.KtorServer.kt</ID>
-    <ID>TooManyFunctions:Server.kt:Server</ID>
-    <ID>TooManyFunctions:ServerSession.kt:ServerSession : Protocol</ID>
   </CurrentIssues>
 </SmellBaseline>