[Ji-One1]

The buyer needs explicit model credentials and guest-shopper instructions. A provider key must not be sent to an unrelated provider merely because another environment variable is present.

What this changes

• Define and validate model name, API key, optional base URL, and non-credential headers.
• Resolve credentials in order: the explicit storefront-tool key/endpoint pair, a matching native provider key, then the known Shopify AI proxy endpoint for that provider.
• Add the Shopify usage tag for proxy requests. Require HTTPS except for loopback HTTP, reject key line breaks, and reject credential-bearing header names.
• Add guest instructions for visible UI actions, blocking overlays, optional-cookie rejection, early friction reports, and stopping after the requested action. They prohibit login, personal/payment details, purchases, and duplicate adds when the outcome is unclear.
• After a blocked checkout navigation, instruct the buyer not to bypass the block or retry checkout. If the objective is unfinished, return through visible storefront UI if needed (browser Back is allowed) and continue; otherwise finish. This is an instruction, not a guarantee of model recovery.
• If a checkout page actually loads, or a page asks for login, personal, contact, address, or payment details, instruct the buyer not to interact: report friction and finish.

Developer impact

Internal tooling only; no published Hydrogen changes or changeset. These are configuration and prompt helpers. This PR does not yet attach a model or run a buyer.

Risk

The operator owns the explicit generic key/endpoint pairing. Instructions alone are not a security boundary. Endpoint validation and fixed validation messages do not provide artifact redaction.

Stack: 11/15. Previous: #4318. Next: #4320.