docs(ai-gateway): document the service-role-only gateway rule

[yardend-wix]

Summary

AI gateway requests must be made server-side with the app's service role (base44.asServiceRole.aiGateway.connection()). Calls from the browser or with a user's token get 403, error code ai_gateway_requires_service_role. The private/workspace user-token exception is a one-line note.

• Authentication Modes: restated to the rule above.
• AiGatewayConnection.token: no longer describes the empty unauthenticated token as a normal case.
• Examples already use asServiceRole, unchanged.

Docs: I didn't run create-docs / push-docs. Published Mintlify already has #305's newer text, which main lacks, so a push from main would regress it. The same wording is applied by hand in base44-dev/mintlify-docs#2569.

Conflict note: #305 rewrites this section. Whichever lands second must keep the service-role paragraph.

🤖 Generated with Claude Code

[github-actions]

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