fix: classify malformed request params as invalid params (#886)

## Summary

Classify typed request deserialization failures as JSON-RPC `Invalid
Params`
instead of `Internal error`.

Fixes #830.

## Changes

- translate `SerializationException` from typed request decoding to
`-32602`
- return a stable `Invalid params` message without serialization
internals
- cover malformed `initialize` requests through session and Streamable
HTTP paths

## Scope and risk

The catch is limited to request parameter deserialization after the
JSON-RPC
method has been resolved. Envelope parsing, unknown methods, handler
failures,
and successful requests keep their existing behavior.

## Verification

```bash
./gradlew :integration-test:jvmTest --tests 'io.modelcontextprotocol.kotlin.sdk.server.ServerSessionInitializeTest'
./gradlew :kotlin-sdk-server:jvmTest --tests 'io.modelcontextprotocol.kotlin.sdk.server.StreamableHttpServerTransportTest'
./gradlew :kotlin-sdk-core:jvmTest --tests 'io.modelcontextprotocol.kotlin.sdk.shared.ProtocolTest'
./gradlew ktlintCheck apiCheck
```

Co-authored-by: Pavel Gorgulov <devcrocod@gmail.com>

kotlin-sdk-server/src/jvmTest/kotlin/io/modelcontextprotocol/kotlin/sdk/server/StreamableHttpServerTransportTest.kt

@@ -47,6 +47,7 @@ import io.modelcontextprotocol.kotlin.sdk.types.ListResourcesResult
 import io.modelcontextprotocol.kotlin.sdk.types.ListToolsResult
 import io.modelcontextprotocol.kotlin.sdk.types.McpJson
 import io.modelcontextprotocol.kotlin.sdk.types.Method
+import io.modelcontextprotocol.kotlin.sdk.types.RPCError
 import io.modelcontextprotocol.kotlin.sdk.types.RequestId
 import io.modelcontextprotocol.kotlin.sdk.types.ServerCapabilities
 import io.modelcontextprotocol.kotlin.sdk.types.Tool
@@ -225,6 +226,36 @@ class StreamableHttpServerTransportTest {
         initResponse.headers[MCP_SESSION_ID_HEADER] shouldBe null
     }
 
+    @Test
+    fun `malformed initialize params return invalid params over streamable http`() = testApplication {
+        val mcpPath = "/mcp"
+
+        application {
+            mcpStreamableHttp(mcpPath, enableDnsRebindingProtection = false) {
+                Server(
+                    Implementation("test-server", "1.0.0"),
+                    ServerOptions(capabilities = ServerCapabilities()),
+                )
+            }
+        }
+
+        val client = createTestClient()
+
+        val response = client.post(mcpPath) {
+            addStreamableHeaders()
+            setBody(
+                """
+                {"jsonrpc":"2.0","id":1,"method":"initialize","params":{"capabilities":{},"clientInfo":{"name":"repro","version":"0.1.0"}}}
+                """.trimIndent(),
+            )
+        }
+
+        response.status shouldBe HttpStatusCode.OK
+        val error = response.body<JSONRPCError>()
+        error.error.code shouldBe RPCError.ErrorCode.INVALID_PARAMS
+        error.error.message.contains("kotlinx.serialization") shouldBe false
+    }
+
     @Test
     fun `request with unsupported protocol version returns an HTTP error`() = testApplication {
         configTestServer()