Reject duplicate feature registration instead of silently overwriting
the existing entry.
## Motivation and Context
`FeatureRegistry.add`/`addAll` silently replaced a feature registered
under the same key and emitted a spurious `list_changed` notification.
Tool/prompt names and resource URIs are unique identifiers in MCP, and
the TypeScript SDK already rejects duplicates at registration time
(`Tool X is already registered`).
- `addTool`/`addPrompt`/`addResource`/`addResourceTemplate` now throw
`IllegalArgumentException` on a duplicate name/URI; the existing
registration stays intact and no notification is emitted.
- `addTools`/`addPrompts`/`addResources` are all-or-nothing: batches
with intra-batch duplicates or conflicts with existing keys are rejected
without registering anything.
- To replace a feature, remove it first (`removeTool` + `addTool`),
matching the TypeScript SDK's remove-then-re-register path. No new
public API.
## How Has This Been Tested?
New `FeatureRegistryTest` unit suite (duplicate rejection, batch
atomicity, no notification on rejection, re-add after remove) plus
integration tests for tools, prompts, resources, and resource templates.
The check also surfaced a real duplicate `addResource` in
`AbstractResourceIntegrationTest`, fixed here. `jvmTest` (all modules),
`apiCheck`, `ktlintCheck`, and `detekt` pass locally.
## Breaking Changes
No public API signature changes (`apiCheck` passes against the existing
dump). Behavioral: registering a duplicate name/URI now throws
`IllegalArgumentException` instead of silently overwriting; remove the
feature first to replace it.
## Types of changes
- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [x] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] 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
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