fix(combo): exempt content_filter from empty-content detection (#7973)

## Problem

When Gemini Flash returns a safety-filtered response (finish_reason:
content_filter, empty content), isEmptyContentResponse() misclassifies
it as a fake-success empty response and returns HTTP 502.  This triggers
the combo fallback chain and account cooldown escalation (5s → 10s →
20s → 40s), even though the response is a legitimate terminal state.

## Root cause

errorClassifier.ts line 14: LEGIT_EMPTY_OPENAI_FINISH only exempts
"length" and "tool_calls".  The "content_filter" finish reason
(mapped from Gemini's SAFETY/PROHIBITED_CONTENT) is not exempted,
so safety-filtered responses are treated as empty content failures.

## Fix

Add "content_filter" to LEGIT_EMPTY_OPENAI_FINISH so safety-filtered
responses pass through as valid (though filtered) completions.

## Testing

- 10/10 unit tests pass (empty-content-stopreason-3572.test.ts)
  including 2 new content_filter test cases
- E2E: hot-patched OmniRoute v3.8.48 on X500, verified the previously
  failing prompt (4.6KB review) now returns valid content instead of
  empty-content 502

Signed-off-by: Minxi Hou <houminxi@gmail.com>

tests/unit/empty-content-stopreason-3572.test.ts

@@ -62,6 +62,24 @@ test("#3572 OpenAI: empty content + finish_reason=stop stays flagged (fake-succe
   );
 });
 
+test("#3572 OpenAI: empty content + finish_reason=content_filter is NOT empty-failure (safety-filtered response)", () => {
+  assert.equal(
+    isEmptyContentResponse({
+      choices: [{ index: 0, message: { content: "" }, finish_reason: "content_filter" }],
+    }),
+    false
+  );
+});
+
+test("#3572 OpenAI: empty content + finish_reason=content_filter (stream chunk) is NOT empty-failure", () => {
+  assert.equal(
+    isEmptyContentResponse({
+      choices: [{ index: 0, delta: { content: "" }, finish_reason: "content_filter" }],
+    }),
+    false
+  );
+});
+
 test("#3572 regression: non-empty content is never flagged", () => {
   assert.equal(isEmptyContentResponse({ content: [{ type: "text", text: "hi" }] }), false);
   assert.equal(