Move raw-protocol server tests to the end-to-end tier

[Firehed]

Summary

Stacked on #681.

Covers the raw-protocol cases of ServerTest on the end-to-end tier, and adds the first variation in client capabilities.

Client capabilities

• Session\ClientCapabilities is what the client declares in initialize, one named field per variation a script needs. Script carries it, defaulting to nothing declared.
• LspClient answers the server's client/registerCapability request with null, as an editor that accepts it would, and fails the test on any other server request.
• Two scripts in transcripts/ cover watched-file registration, since the exchange itself is the point: one declares dynamic registration support and locks the server's registration and the client's answer; one declares nothing and locks that none is sent.

Raw-protocol tests

• LifecycleTest gains: exit without shutdown exits 1, closed input exits 1, a request before initialize gets -32002, a request after shutdown gets InvalidRequest and exit still returns 0.
• MessageErrorTest covers: an unknown method gets MethodNotFound; a frame that is not JSON gets ParseError at a null id; a frame that is JSON but not a valid request gets InvalidRequest at its own id. Each checks the session ends cleanly afterward.
• Client\ErrorCode holds the specification's codes for these assertions, since nothing in the tier may depend on src/.

Originals

• The two watched-file registration tests in ServerTest are removed: they used Server::forProject and the container, and the scripts cover them.
• The other ServerTest cases build Server directly and test the read loop, so they stay.

Coverage: unchanged.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.73%. Comparing base (7f69765) to head (ee0397c).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #682   +/-   ##
=========================================
  Coverage     99.73%   99.73%           
  Complexity     1900     1900           
=========================================
  Files           135      135           
  Lines          4873     4873           
=========================================
  Hits           4860     4860           
  Misses           13       13           

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