test: snapshot the MCP tool contract and exercise registration through an in-memory server (#13)

First of the refactoring series: the instruments that let every later PR
prove it changed nothing a client can see.

## Change

- `Tests/EventKitMCPTests/Contract/tool-contract.json` is
`ToolRegistry.allTools()` encoded with sorted keys. The new
`toolContractMatchesSnapshot` test compares against it and names the
first differing line; `UPDATE_TOOL_CONTRACT=1 swift test --filter
toolContractMatchesSnapshot` regenerates it. A changed snapshot in a
diff is the signal that the PR title needs `feat`, `fix` or `!`.
- `ServerRegistrationTests` drives `ToolRegistry.registerHandlers`
through the SDK's `InMemoryTransport` and a real `Client`. It pins the
read-only listing and the read-only call gate end to end, which nothing
reached before.
- `inputContracts` is deleted. Its substring checks are a strict subset
of the snapshot.
- `Package.swift` excludes the `Contract` directory from the test target
so SwiftPM does not warn about an unhandled file.

The snapshot also settles a question for later work: every optional
input property is already advertised as `["<type>", "null"]`, so clients
may send `null` today.

## Verification

- `swift test`: 31 library tests and 141 server tests pass (139 before,
+3 new, −1 deleted).
- `swift build -c release` is clean.

## Mutation record

| Mutation | Failing tests |
|---|---|
| `time filter` → `time filters` in the query tool description |
`toolContractMatchesSnapshot`, reported as `tool-contract.json:8` with
both lines |
| overview declared `destructiveHint` instead of `readOnlyHint` |
`toolContractMatchesSnapshot`, `readOnlyTools`, `readOnlyServer` |

Both reverted before commit.

## Release impact

None. Tests and a test-target manifest exclude only.

Author: k3KAW8Pnf7mkmdSMPHz27

Package.swift

@@ -72,7 +72,8 @@ let package = Package(
                 .product(name: "MCP", package: "swift-sdk"),
                 .product(name: "JSONSchema", package: "swift-json-schema")
             ],
-            path: "Tests/EventKitMCPTests"
+            path: "Tests/EventKitMCPTests",
+            exclude: ["Contract"]
         )
     ],
     swiftLanguageModes: [.v6]