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

[k3KAW8Pnf7mkmdSMPHz27]

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.

[eventkit-tap-bumper]

🎉 This PR is included in version 3.0.0 🎉

The release is available on GitHub release

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