Move context-filtering completion tests to the end-to-end tier

[Firehed]

Moves the context-filtering completion tests out of CompletionHandlerIntegrationTest into completion-context-filtering.php: a comment, member access in a comment, a heredoc (variables only), and the position after a finished statement.

Part of #694.

Unit tests. CompositeCompletionSourceTest now covers the composite's early returns and its None classification:

• In a comment, and after a finished statement, nothing is offered, even with every symbol kind available.
• Member access in a comment offers nothing even when members resolve: the comment check comes first.
• In an interpolated string, only variables are offered, even inside a call.

ContextDetectorTest and CompletionClassifierTest already own which positions are comments, strings, heredocs, or unclassified.

Script. The finished-statement case types (); after an existing marker rather than opening inline PHP. The plain-comment case's fixture has no word before the cursor, so it would answer nothing even without the early return; member access in a comment is the case that needs it.

A moved position. The deleted "unknown context" test put its cursor before the ; of $x = 1;, where the prefix is 1. Its name says after the statement, so both replacements put the cursor there.

Coverage. CompositeCompletionSource is now fully covered by its own unit test.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.75%. Comparing base (237978d) to head (9919c42).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #719      +/-   ##
============================================
+ Coverage     82.71%   82.75%   +0.04%     
  Complexity     1901     1901              
============================================
  Files           135      135              
  Lines          4876     4876              
============================================
+ Hits           4033     4035       +2     
+ Misses          843      841       -2     

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