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

conformance-test/src/main/kotlin/io/modelcontextprotocol/kotlin/sdk/conformance/ConformanceTools.kt

@@ -45,7 +45,6 @@ internal const val WAV_BASE64 = "UklGRiYAAABXQVZFZm10IBAAAAABAAEAQB8AAAB9AAACABA
 
 private val logger = KotlinLogging.logger {}
 
-@Suppress("LongMethod")
 fun Server.registerConformanceTools() {
     // 1. Simple text
     addTool(