Remove CompletionHandlerIntegrationTest

[Firehed]

Deletes CompletionHandlerIntegrationTest, the last of its groups. Part of #694.

Parse once. testCompletionParsesTheDocumentOnce checked one parse per completion request at three positions. With memoization disabled, only the variable-prefix case failed; the member- and static-access positions parse once whether or not the memo works. ServerTest::testParsesAreScopedToOneMessageOnTheRequestPath already sends two variable-prefix completions through Server::forProject and asserts three parses for the open and both requests; its docblock now says why that position was chosen. MemoizingSyntaxSourceTest owns the memo itself.

Wrapped implements. The skipped test pointed at #310, which is closed as not planned; it ran nothing. #720 now tracks the bug with its reproduction and the current output, so the test and its fixture WrappedImplementsCompletion.php go.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.75%. Comparing base (1d343e1) to head (f15b7ad).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #721   +/-   ##
=========================================
  Coverage     82.75%   82.75%           
  Complexity     1901     1901           
=========================================
  Files           135      135           
  Lines          4876     4876           
=========================================
  Hits           4035     4035           
  Misses          841      841           

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