feat(product): identify this crate's backend traffic as opencompany - #513

The prior finding about the product header on hub_identity's /auth/me call is in a different file and is not addressed by this diff — noting it as not fixable from this file's review scope. For src/harness/provider.rs itself: the change attaches the x-sdk-name: opencompany product header to HostedProvider::invoke (the managed chat-completions path) and to request_plan only when provider == _The code index is behind this pull request (indexed at 8b2bf54`), so retrieved context may be out of date._

No findings.

Fixed since the last review

• Cover the product header on hub_identity's /auth/me call