feat(server): auto-install ContentNegotiation with McpJson (#664) (#665)

Users currently must manually `install(ContentNegotiation) {
json(McpJson) }` before calling `mcp()`,
`mcpStreamableHttp()`, or `mcpStatelessStreamableHttp()`. Forgetting
this or using the default `json()`
instead of `json(McpJson)` causes silent serialization failures at
runtime (e.g., explicit null fields in JSON-RPC
responses). This PR makes the SDK install it automatically.

Fixes #664

## Changes

- **Core feature** — `mcp()`, `mcpStreamableHttp()`, and
`mcpStatelessStreamableHttp()` now auto-install `ContentNegotiation`
configured with `McpJson`. If the user already installed
`ContentNegotiation`, a warning is logged instead of crashing with
`DuplicatePluginException`.

- **`KtorServerHelpers.kt`** (new) — `installMcpContentNegotiation()`
internal helper with `pluginOrNull` guard and warning
- **`KtorServer.kt`** — calls the helper from all three entry points
- **`build.gradle.kts`** — adds `ktor-server-content-negotiation` and
`ktor-serialization` as `implementation` deps to `kotlin-sdk-server`

- **Removed manual installs** from `ConformanceServer.kt` and
`AbstractStreamableHttpIntegrationTest.kt`.

- **Test infrastructure** — replaced `slf4j-simple` with Logback and use
`ListAppender` (`LogCapture` utility) for deterministic log assertion.
Added `logback-test.xml` matching previous log levels.

- **`StreamableHttpServerConfigurationTest.kt`** (new) — verifies the
warning for all three entry points: `mcpStreamableHttp`,
`mcpStatelessStreamableHttp`, and `mcp` (SSE).

- **README.md** — removed manual `ContentNegotiation` boilerplate from
examples, documented automatic installation, added CORS configuration
snippet for browser-based clients.

- **CI** — added `ktlintCheck` to the Linux build matrix job.


## How Has This Been Tested?

Unit/integration tests. Conformance tests updated

## Breaking Changes

No

## Types of changes
- [x] Bug fix (non-breaking change which fixes an issue)
- [x] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [x] Documentation update

## Checklist

- [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

integration-test/src/jvmTest/kotlin/io/modelcontextprotocol/kotlin/sdk/integration/streamablehttp/AbstractStreamableHttpIntegrationTest.kt

@@ -1,17 +1,13 @@
 package io.modelcontextprotocol.kotlin.sdk.integration.streamablehttp
 
-import io.ktor.serialization.kotlinx.json.json
-import io.ktor.server.application.install
 import io.ktor.server.cio.CIOApplicationEngine
 import io.ktor.server.engine.EmbeddedServer
 import io.ktor.server.engine.embeddedServer
-import io.ktor.server.plugins.contentnegotiation.ContentNegotiation
 import io.modelcontextprotocol.kotlin.sdk.server.Server
 import io.modelcontextprotocol.kotlin.sdk.server.ServerOptions
 import io.modelcontextprotocol.kotlin.sdk.server.mcpStreamableHttp
 import io.modelcontextprotocol.kotlin.sdk.types.GetPromptResult
 import io.modelcontextprotocol.kotlin.sdk.types.Implementation
-import io.modelcontextprotocol.kotlin.sdk.types.McpJson
 import io.modelcontextprotocol.kotlin.sdk.types.PromptArgument
 import io.modelcontextprotocol.kotlin.sdk.types.PromptMessage
 import io.modelcontextprotocol.kotlin.sdk.types.Role
@@ -56,9 +52,6 @@ open class AbstractStreamableHttpIntegrationTest {
             host = URL,
             port = PORT,
         ) {
-            install(ContentNegotiation) {
-                json(McpJson)
-            }
             mcpStreamableHttp { mcpServer }
         }