fix: wire persisted provider configuration into runtime

[DaBear987]

Wires persisted provider configuration from runtime settings into actual provider execution. Provider kind is authoritative when explicitly configured, with regression coverage for Anthropic, xAI, OpenAI, DashScope, and local OpenAI-compatible routing. Validation completed with cargo fmt --check, runtime/API/CLI test suites, and git diff --check.

[1716775457damn]

Good change — making provider kind authoritative removes the ambiguity of a persisted config silently overriding the routing. One question for clarity: with kind authoritative, what wins against an explicit runtime hint, e.g. a --provider flag or a model prefix on the command line? If the stored kind always wins regardless of the current invocation, a user doing a one-off provider switch could be surprised. Either a short precedence note in the docs or a test covering flag-vs-persisted precedence would make the intent unambiguous.