Prepare package slicing by strict namespace refactoring into Client, Schema, and Server

[chr-hertel]

Proposal to rework namespaces in favor of slicing mcp/sdk package, see #489

1. Moved everything below Mcp\Client, Mcp\Schema, or Mcp\Server
2. Distributed Mcp\Exception into component specific namespaces

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Next steps would be:

• introduce composer setups per component
• configure split.sh
• ???

[Copilot]

🟡 Changes recommended

Numerous tests expect exceptions from the wrong slice, several examples reference invalid namespaces, and Deptrac is not enforced by CI.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

Refactors the SDK into distinct Client, Schema, and Server namespaces in preparation for package slicing.

Changes:

• Moves facades, capabilities, events, exceptions, and shared protocol types into slice-specific namespaces.
• Updates tests, examples, documentation, and autoloading references.
• Adds Deptrac boundaries and inspector snapshots.

File summaries

File group	Description
src/Client/**	Refactors client APIs and exceptions.
src/Schema/**	Consolidates shared protocol types and schema exceptions.
src/Server/**	Refactors server APIs, capabilities, events, and exceptions.
tests/Unit/**	Migrates unit tests and fixtures.
tests/Integration/**	Updates integration imports and fixtures.
tests/Conformance/**	Updates conformance entry points.
tests/Inspector/**	Adds structured-result snapshots.
examples/**	Migrates client and server examples.
docs/**, README.md	Updates public namespace guidance.
composer.json, deptrac.yaml, phpstan.dist.neon, .gitignore	Configures architecture tooling and relocated paths.

Review details

Suppressed comments (1)

docs/client/errors.md:24

• Client requests throw Mcp\Client\Exception\RequestException. Catching the server-slice sibling will not handle $client->callTool() failures.

• Files reviewed: 299/423 changed files
• Comments generated: 43
• Review effort level: Balanced

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[soyuka]

For splitsh this is how we did on API Platform: https://github.com/api-platform/core/blob/main/.github/workflows/release.yml#L21-L50

Pretty straightforward, not sure how it works on Symfony but AFAIR it was a bit more complex :p. Note that testing may also be a bit of a headache if we allow wider version constraint on one of these packages... +1 to keep tests global to this repo (sometimes its interesting to have component specific tests especially that tests also get published along the source code though it makes things more complicated).