fix(sse): treat antigravity empty completions with a normal stop as valid 200s (#14160) - #14243

What

An empty (but successful) completion from antigravity's Gemini is treated as a provider failure: the client gets 502 Provider returned empty content and the model is locked out on that account. The reporter sees ~400 of these a day from a single observer prompt the model legitimately answers with no text, which keeps most of a 22-connection pool excluded and starves unrelated clients (their words: a cheap harmless request was starving the expensive ones).

The fake-success guard exists for free-tier/scraping providers whose failure mode is an empty 200 shell (#13461). Antigravity is Google's first-party Gemini API: an empty candidate with finishReason: STOP is a real result, and retrying the same prompt on the next account returns the same empty answer anyway.

Changes

• isEmptyContentResponse() gains an optional provider and a TRUSTED_EMPTY_STOP_PROVIDERS allowlist (same pattern as the provider-allowlisted classifyFakeSuccessBody from [BUG] Gateway returning bad response #13461). On antigravity, an empty completion with a normal terminal stop reason (openai stop, claude end_turn) is no longer flagged.
• The non-streaming leg passes its provider through, so these completions flow to the client as a valid 200 with empty content instead of a synthetic 502. No failure accounting, no model lockout, and no retrying the same prompt on the next account to get the same empty answer.

Unchanged: every other provider keeps the guard, including on empty stop completions; antigravity responses with no terminal stop reason are still flagged; the existing legit-empty carve-outs (length, tool_calls, content_filter, max_tokens, tool_use) are untouched.

Verification

• New test tests/unit/14160-antigravity-empty-stop-completion.test.ts, 5 cases: antigravity empty+stop and empty+end_turn pass through; untrusted providers and the no-provider call keep flagging them; an antigravity empty shell with no finish reason is still flagged; existing carve-outs unchanged.
• Red-green checked against base: with the src changes reverted and only the test kept, the 2 antigravity cases fail and the 3 controls pass; with the fix, 5/5 pass.
• Adjacent suites: 199/199 across the error-classifier, non-streaming-leg, empty-content, fake-success, and silent-empty web-provider suites (incl. combo-empty-content-failover-5085, diagnostics-fake-success-13461, copilot-m365-web-silent-empty-7858, zai-web-silent-empty-repro).
• prettier --check, eslint, typecheck:core clean on all touched files.

Out of scope: the streaming variant (#13600) is a different code path (streamEmptyChoices.ts) and is not touched here. If you'd rather have the passthrough behind a settings toggle, happy to add one.

Fixes #14160