refactor: explicit locked detekt config (#729)

Migrate detekt to a convention plugin in `buildSrc`, switch the project
to a fully explicit YAML config (`buildUponDefaultConfig = false`), fix
the real findings that this stricter setup revealed, and remove
`@Suppress` annotations that became unnecessary after the rule changes.

## Motivation and Context
- Explicit visibility into the rule set
- Lock against detekt version upgrades

## Breaking Changes
None

## Types of changes
- [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)
- [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

test-utils/src/jvmMain/kotlin/io/modelcontextprotocol/kotlin/test/utils/Retry.kt

@@ -16,7 +16,6 @@ import java.util.Optional
 @ExtendWith(RetryExtension::class)
 public annotation class Retry(val times: Int = 3, val delayMs: Long = 1000)
 
-@Suppress("ForbiddenVoid") // needed for Java API interop
 public class RetryExtension : InvocationInterceptor {
     override fun interceptTestMethod(
         invocation: Invocation<Void>,

test-utils/src/jvmMain/kotlin/io/modelcontextprotocol/kotlin/test/utils/TypeScriptRunner.kt

@@ -18,7 +18,6 @@ public object TypeScriptRunner {
      * @param logPrefix Prefix for log lines
      * @return The started Process
      */
-    @Suppress("LongParameterList")
     public fun run(
         typescriptDir: File,
         scriptPath: String,
@@ -54,7 +53,6 @@ public object TypeScriptRunner {
      * @param typescriptDir The directory containing package.json
      * @throws RuntimeException if npm install fails
      */
-    @Suppress("ReturnCount")
     public fun installDependencies(typescriptDir: File) {
         require(typescriptDir.isDirectory()) { "Type script directory does not exist: ${typescriptDir.absolutePath}" }
         println("Installing TypeScript dependencies in ${typescriptDir.absolutePath}")

test-utils/src/jvmMain/kotlin/io/modelcontextprotocol/kotlin/test/utils/processUtils.kt

@@ -14,7 +14,6 @@ import kotlin.time.Duration.Companion.seconds
  */
 public fun killProcessOnPort(port: Int) {
     val killCommand = if (isWindows) {
-        @Suppress("MaxLineLength")
         listOf(
             "cmd.exe",
             "/c",