Define provider adapter hook for browser connector bridge

[chubes4]

Problem

Issue #542 needs WP Codebox to provide a generic server-side bridge for browser Playground AI/provider calls. The bridge can own connector-scoped authorization and redacted audit metadata, but provider execution may need a generic extension point so WP Codebox does not become coupled to a specific provider, Studio Web, or Data Machine.

Desired shape

After the bridge contract is implemented, define or validate the provider execution hook needed by callers/providers:

WP Codebox browser connector request
  -> server-side connector auth resolution
    -> generic provider adapter hook/filter
      -> provider request

The hook should receive connector/provider/model/session/caller context plus a generic request object, and return a redacted generic response envelope.

Acceptance criteria

• Provider execution extension point is generic and documented.
• It is connector-scoped and auditable by provider, model, connector, session/job, and caller context.
• It fails closed when no provider adapter handles the request.
• It does not mention Studio Web or Data Machine in WP Codebox core.
• It does not expose raw provider keys to browser JS, browser Playground PHP, artifacts, or diagnostics.

Related

• Parent secret bridge tracker: Add server-side secret bridge for browser Playground AI calls #542