Move response cap completion tests to the end-to-end tier

[Firehed]

Moves the response cap tests to the end-to-end tier and merges CompletionCapFloodTest into it.

Part of #694.

Script. completion-response-cap.php completes the prefix s inside namespace Fixtures\Completion (new fixture src/Completion/ResponseCap.php). Core PHP's s* functions overflow the cap, so the list is reported incomplete. The file's own summarizeTotals still survives: it ranks as a current-namespace symbol, while by name well over 100 core functions (str*, stream_*, sprintf, …) sort before it. Sorting the cap by label alone makes the script fail. This is the flood test's point with real data instead of a generated classmap, and it needs no optional extension.

The flood test also covered built-in functions beating workspace classes in global and call contexts. Those pieces stay unit-tested (ReferenceKindTest orders the kinds, ReferenceResolverTest assigns them, SymbolCandidatesTest writes the prefix), and every context reaches the prefix through the same SymbolCandidates::find.

Unit tests.

• ReferenceKindTest is new. The unit suite did not cover ReferenceKind::priority(), the ranking that decides which symbols survive the cap. It pins the order and that no two kinds tie.
• CompletionHandlerTest already covers the cap itself: untouched under the limit, sorted by sortText ?? label, truncated, and flagged incomplete over it.

New vocabulary. ReportsIncomplete() checks the list's isIncomplete flag.

Removed.

• Both cap tests from CompletionHandlerIntegrationTest, and the setup they alone needed (a seeding helper and a handler factory taking its own source).
• CompletionCapFloodTest.
• The flood() method of Namespacing/AbsoluteNavigation.php, whose marker only the deleted navigation cap test used.

Coverage. ReferenceKind and CompletionHandler are fully covered by their own unit tests.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.71%. Comparing base (b52e2a1) to head (50e6196).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #718      +/-   ##
============================================
+ Coverage     82.52%   82.71%   +0.18%     
  Complexity     1901     1901              
============================================
  Files           135      135              
  Lines          4876     4876              
============================================
+ Hits           4024     4033       +9     
+ Misses          852      843       -9     

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