fix(antigravity): give the executor and the probe one client identity

[kittors]

The problem

Both halves of this provider call CloudCode, and each carried its own User-Agent literal:

caller	advertised
request executor	antigravity/1.104.0 darwin/arm64
account status probe	antigravity/1.11.5 windows/amd64

One account looked like two different clients depending on which half of the system was asking, and both were far behind the real Antigravity release (4.3.0).

CloudCode gates its answer on that header. The probe, on the older of the two, was served a reduced model set and a coarser quota view than the account actually had — which is why three model families reported one identical percentage and one identical reset time down to the second. Upstream they were a single bucket that the older client could not see broken down.

The fix

Both now derive from antigravity.ClientUserAgent, built from a single ClientVersion.

The OAuth identity deliberately stays separate: token and userinfo calls go to Google's OAuth endpoints rather than CloudCode, and google-api-nodejs-client is the correct client for those. A test pins that the two must not collapse into one.

Tests pin properties, not strings

• ClientVersion may not slide below the known floor (4.3.0)
• ClientUserAgent must carry the version, in the vscode/1.X.X (Antigravity/…) form the real client sends
• OAuth and CloudCode identities must stay distinct
• The executor must resolve to the shared constant, so a local literal cannot be reintroduced

Per-account user_agent overrides still win — that path is untouched.

What I did not do, and why

Folding this into internal/identityfingerprint. That package is a learning system: it derives fingerprints from observed client request headers, and it is currently codex-specific (CodexProfileKey, classifyCodexUserAgent, classifyCodexOriginator). A server-initiated probe has no client request to learn from, so the fit is poor. #884 is also actively reworking those exact files, and this change would collide with it.

If #884 lands and someone wants antigravity in that system afterwards, ClientVersion is the single place to move.

Verification

go build ./..., go vet ./internal/... ./sdk/..., gofmt all clean; full go test ./... green locally.

Note this path cannot be verified against production: the host has no antigravity accounts (ai_account_status holds only xai, codex and claude), so this rests on the tests plus the fact that the upstream client itself sends this header.

🤖 Generated with Claude Code