fix: drop redundant borrow in ai_provider Authorization header

[andrewbudge]

The workspace clippy gate (cargo clippy --workspace -- -D warnings) fails on current stable (clippy >= 1.97) with useless_borrows_in_formatting:

error: redundant reference in `format!` argument
  --> crates/oxo-flow-web/src/ai_provider.rs:106
   |
   | .header("Authorization", format!("Bearer {}", &self.api_key))
   |                                                ^^^^^^^^^^^^^ help: remove the redundant `&`: `self.api_key`

This is pre-existing on main (landed in #45); newer clippy tightened the lint, so the gate now blocks every PR built on this toolchain. Fix passes the field by value. No behavior change.