fix(index): use GEMINI_API_KEY for google provider startup validation

[Nivesh353]

Problem

The startup validation in index.ts checked GOOGLE_API_KEY for the
google: provider, but pi-ai's env-api-keys.js uses GEMINI_API_KEY
for actual API calls.

This caused two issues:

• Users setting only GEMINI_API_KEY got a crash at startup:
  Error: GOOGLE_API_KEY environment variable is not set
• Users had to set both keys to get the google: provider working,
  with no indication that two separate keys were needed

Fix

Changed the startup env var check for the google: provider from
GOOGLE_API_KEY to GEMINI_API_KEY, aligning with what pi-ai uses
for authentication.

Impact

• One-line change in index.ts
• Users now only need GEMINI_API_KEY for the google: provider
• No functional change to API behavior — only the startup guard is updated

[shreyas-lyzr]

The fix is correct. The rest of the codebase — line 430 in index.ts, Documentation.md (the provider table and env var reference) — all consistently use GEMINI_API_KEY for the google provider. The startup guard was the only outlier, and this one-line change brings it into alignment. No issues found.