fix(providers): surface real Antigravity upstream error detail (#13591)

[diegosouzapw]

Refs #13591

Not covered here: the suspected root trigger (omr_skill_* schema rejections causing the
400s in the first place) is tracked separately at #13583, already fixed by #11857 on
release/v3.8.51. This PR only fixes the observability half — surfacing the real upstream
detail instead of the opaque wrapper text — which is how the owner explicitly scoped the
issue in their own comment.

Root cause (short)

Antigravity double-wraps its own errors. buildAntigravityUpstreamError()
(open-sse/executors/antigravityUpstreamError.ts) replaced error.message with the generic
"Antigravity upstream error (400)" template and buried the real Gemini-dialect detail one
level down, under upstream_details. chatCore.ts's shared failure-classification path
(providerFailure: block, both streaming and non-streaming legs) re-parses that
already-wrapped body with the generic parseUpstreamError(), which only reads the outer
error.message — so the generic text is what ends up in createErrorResult().error, the
persisted call_logs/proxy_logs, and the [ProxyEgress] antigravity status=error line.
This is why gemini (the plain API-key/OAuth provider) shows the real field-path message and
antigravity doesn't: the gemini executor never pre-wraps non-ok responses, so the raw
Google body survives untouched to the same parser.

Fix

buildAntigravityUpstreamError() now extracts the real upstream error.message from the
parsed upstream_details (when present) and composes the final error.message as
"Antigravity upstream error (<status>): <real detail>" instead of the bare templated
string. This fixes both consumers for free — the client-facing HTTP response and every
internal log/telemetry path that reads error.message — with no changes needed in
chatCore.ts or parseUpstreamError(). The existing geo-blocked branch (its own explicit,
more useful hint text) is left untouched, and a non-JSON upstream body (e.g. an HTML error
page) still falls back to the generic templated message without throwing.

Regression test

tests/unit/antigravityUpstreamError.test.ts (new file, 3 cases):

RED (on unfixed code):

✖ issue #13591: parseUpstreamError surfaces the real upstream detail for an Antigravity-wrapped 400
  AssertionError [ERR_ASSERTION]: buildAntigravityUpstreamError should surface the real upstream
  detail directly in error.message

GREEN (after the fix):

✔ issue #13591: parseUpstreamError surfaces the real upstream detail for an Antigravity-wrapped 400 (1570.882722ms)
✔ issue #13591: geo-blocked branch keeps its explicit hint message untouched (985.580498ms)
✔ issue #13591: non-JSON upstream body falls back to the generic templated message without throwing (34.035571ms)
ℹ tests 3
ℹ pass 3
ℹ fail 0

Run command: DATA_DIR=$(mktemp -d) timeout 300 node --import tsx/esm --test --test-force-exit tests/unit/antigravityUpstreamError.test.ts

Gates run

• npx eslint --suppressions-location config/quality/eslint-suppressions.json open-sse/executors/antigravityUpstreamError.ts tests/unit/antigravityUpstreamError.test.ts — exit 0
• npm run check:open-sse-typecheck — OK, 0 pre-existing errors
• node scripts/check/check-file-size.mjs — OK on touched files (1 unrelated pre-existing violation in open-sse/utils/stream.ts, not touched by this PR)
• node scripts/check/check-complexity.mjs — OK (2824 violations vs baseline 3218)
• node scripts/check/check-cognitive-complexity.mjs — OK (1276 violations vs baseline 1437)
• node scripts/check/check-test-discovery.mjs — OK, new test file discovered
• Existing Antigravity error-shape tests re-run and still green:
  • tests/unit/antigravity-geoblock-resilience.test.ts (16/16 pass)
  • tests/unit/agy-gemini-400-3229.test.ts (pass)
  • tests/unit/antigravity-streaming-error-body-sanitized-2461.test.ts (pass)

Existing tests aligned

None — no existing assertion encoded the old buggy contract; all three existing test files
above exercise shapes unaffected by this change (binary/non-JSON bodies, the geo-blocked
branch, and pass-through behavior for non-Antigravity providers) and pass unmodified.