fix(ai): antigravity-auth does not apply the blank-credential rule — whitespace keyring tokens are accepted and sent

Summary

The repo's auth bridges share one rule, stated verbatim in the #1763 fix
(commit 0dd051a394, PR #1764): "A blank value is no credential (the
vibe-auth rule): trim, and treat whitespace-only as absent." vibe-auth.ts
trims at every tier (:146 env, :154 file, :160 keyring), and #1763 extended
the rule to commandcode-auth.ts and claude-code-auth.ts with tests.

packages/ai/src/providers/antigravity-auth.ts — the fourth first-party
auth bridge — was not touched and applies the rule nowhere. Verified by
reading on main 2981dfd838:

Evidence

1. Whitespace-only refresh token passes the shape check and is POSTed.
   antigravity-auth.ts:115 — if (!creds.token?.refresh_token) return reject(...) rejects only missing/empty values. A refresh token of
   " " passes, and refresh() sends it to
   https://oauth2.googleapis.com/token as refresh_token (:252), where
   it fails as an opaque invalid_grant instead of being read as "no
   credential stored".

2. Whitespace-only access token is cached and sent as a Bearer.
   loadToken (:308): if (creds.accessToken && creds.expiryMs - Date.now() > 60_000) — a whitespace-only access_token from the
   keyring is truthy, so with a future expiry it is cached and returned;
   keyringAuth then emits authorization: \Bearer ${token.accessToken}``
   (:335) — literally a Bearer token of spaces, the exact failure fix(ai): commandcode + claude-code auth bridges accept whitespace-only credentials — a blank env var shadows commandcode's valid login file, and both bridges send a Bearer header of spaces #1763's
   title names for the sibling bridges.

3. Whitespace-only access token from the token endpoint is accepted.
   refresh() (:274): if (!parsed.access_token) rejects only
   missing/empty; a whitespace-only access_token in a 200 response is
   cached and later sent as a Bearer per (2).

4. No test pins any of this. packages/ai/test/provider/antigravity-auth.test.ts
   covers only refresh bounding (fix(ai): Antigravity token refresh fetch has no deadline, response body unbounded — check:bounded-http is red on main #1469) and client-secret candidates
   (antigravity: client-secret extraction (first GOCSPX match) stale for agy 1.4.11 — refresh fails invalid_client #1633); grep for trim|whitespace|blank in that file: 0 hits. The
   sibling bridges each gained explicit whitespace tests in fix(ai): commandcode + claude-code auth bridges accept whitespace-only credentials — a blank env var shadows commandcode's valid login file, and both bridges send a Bearer header of spaces #1763
   (claude-code-auth.test.ts, commandcode-auth.test.ts).

Why it matters

The keyring entry is written by an external binary (agy) and read back
verbatim; a corrupted or placeholder entry currently degrades into a
confusing upstream auth failure (or a spaces-only Bearer) instead of the
bridge's own clear "credential unavailable / unexpected shape" errors.
Consistency also matters for the doctrine in #1717: all *-direct bridges
should treat blank credentials identically, so fall-through and fail-closed
behavior is predictable across providers.

Acceptance criteria

• Keyring parsing treats whitespace-only refresh_token as an
  unexpected-shape/absent credential (same rejection path as a
  missing one), and trims stored tokens before use.
• A whitespace-only keyring access_token is never cached or sent;
  loadToken falls through to refresh.
• refresh() rejects a whitespace-only access_token in the token
  endpoint response (treated like "returned no access_token") and
  trims accepted tokens.
• New tests in packages/ai/test/provider/antigravity-auth.test.ts
  pin all three cases, mirroring the fix(ai): commandcode + claude-code auth bridges accept whitespace-only credentials — a blank env var shadows commandcode's valid login file, and both bridges send a Bearer header of spaces #1763 sibling tests; existing
  fix(ai): Antigravity token refresh fetch has no deadline, response body unbounded — check:bounded-http is red on main #1469/antigravity: client-secret extraction (first GOCSPX match) stale for agy 1.4.11 — refresh fails invalid_client #1633 tests stay green.
• bun run check:bounded-http / gate state not regressed by the
  change (note: the three quota-era gates are already red on main
  under fix(ci): three gates red on main — PR #1722's quota subcommand landed 16 undispositioned sites, and turso bypasses the PR's own bounded reader #1724 — do not conflate).

Filed by new-issue-creator-opencode; sibling of closed #1763 (same rule,
bridge not covered there — deduped against open/closed search for
"blank credential whitespace" and "antigravity auth", 2026-11-05).