feat(gemini-adapter): prefer HTTP API when GEMINI_API_KEY is set

[abigail-atheryon]

Summary

The OAuth tier hits hard daily quotas on gemini-2.5-pro after a handful of calls (verified empirically — repeated /benchmark-models runs trip "You have exhausted your capacity on this model"). The CLI's stream-json output also reports 0/0 tokens for OAuth runs, so cost numbers in the benchmark table were always $0.0000 — even when real billing was happening on paid accounts.

This patch prefers the Generative Language HTTP API directly when GEMINI_API_KEY (or GOOGLE_API_KEY) is set:

• Paid quotas (no daily cap surprises mid-benchmark)
• Full usage data, including thoughtsTokenCount for reasoning models — folded into output token count so cost estimation matches the real bill
• No CLI dependency on the API-key path

CLI fallback retained for free-OAuth users with no API key, so /benchmark-models still works out of the box on a fresh install.

Stacked on

Stacks on top of #1362 (the OAuth path-detection fix). Targets main; will rebase cleanly once #1362 lands.

Auth precedence

1. GEMINI_API_KEY → HTTP path (preferred)
2. GOOGLE_API_KEY → HTTP path
3. ~/.gemini/oauth_creds.json or ~/.config/gemini/ → CLI path

Verification

Before (OAuth, gemini-2.5-pro):
```
gemini-2.5-pro 275.6s 0→0 $0.0000 ERROR rate_limit
```

After (HTTP, GEMINI_API_KEY set):
```
gemini-2.5-pro 12.7s 23→1294 $0.0065 OK
```

Same prompt, same gstack-model-benchmark invocation. Only GEMINI_API_KEY was added.

Test plan

• HTTP path returns real tokens + cost for gemini-2.5-pro and gemini-2.5-flash
• CLI fallback still works when no API key is set (verified with key unset)
• OAuth-only users see unchanged behavior
• Reasoning ("thoughts") tokens correctly attributed to output for cost
• Maintainer to verify on a Vertex/non-personal account if relevant

Notes

The HTTP rewrite is roughly the same shape as the existing GPT adapter (CLI shell-out) but uses fetch() for the API-key path. No new deps, no test infra changes.

🤖 Generated with Claude Code

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[garrytan]

Thanks @abigail-atheryon — closing as deferred. Gemini HTTP API preference is a substantial behavioral change to the gemini adapter (112 lines). Companion #1362 (oauth path detect) shipped in a prior wave; the HTTP API switch warrants focused review on its own. Happy to revisit.