fix(sse): green PR #7223 CI — complexity ratchet + stale test expectations

- Extract applyChatCompletionsOnlyQuirks() and resolveInitiatorHeader()
  out of GithubExecutor.transformRequest()/buildHeaders() so the two
  methods drop back under the complexity/cognitive-complexity ratchets
  (2058/891 -> 2056/890, matching the frozen baseline). No behavior
  change — same guards, just relocated.
- Update 4 pre-existing unit tests that hard-coded now-native claude-*
  Copilot ids (claude-sonnet-4.5/4.6) to exercise the /chat/completions
  legacy path via an unregistered id (claude-sonnet-4), matching the
  sibling test already using that pattern. These ids now intentionally
  route to the native /v1/messages shim added by this PR, which
  correctly skips the /chat/completions-only workarounds these tests
  were built to verify — the native path's own coverage lives in
  github-copilot-claude-native-messages.test.ts.
- Split the routing invariant test (copilot-gemini-claude-route-no-responses.test.ts)
  into a Claude case (expects /v1/messages) and a Gemini case (still
  expects /chat/completions), reflecting the intentional routing change.

tests/unit/executor-github.test.ts

@@ -161,7 +161,13 @@ test("GithubExecutor.transformRequest sanitizes Anthropic-shape content parts (t
     ],
   };
 
-  const result = executor.transformRequest("claude-sonnet-4.6", body, true, {});
+  // Use an unregistered claude-* id (not "claude-sonnet-4.6"/etc.) so
+  // getModelTargetFormat("gh", ...) resolves to null and this stays on the
+  // /chat/completions path this test targets. Registered claude-* ids now
+  // carry targetFormat:"claude" (native /v1/messages — port of
+  // decolua/9router#2608, see github-copilot-claude-native-messages.test.ts)
+  // and intentionally skip this sanitization.
+  const result = executor.transformRequest("claude-sonnet-4", body, true, {});
 
   // user message keeps text + image_url parts untouched
   assert.equal(result.messages[0].content[0].type, "text");