[Bug] `--provider openai-api` fails with credential selection error when `default_provider` is a named profile

Summary

When [provider].default_provider in config.toml is set to any custom named profile (e.g., company-gateway, my-local-proxy, etc.), explicitly overriding it via the CLI with --provider openai-api fails. The CLI aborts with:
Failed to select the credential route for --provider openai-api: Provider OpenRouter does not support OAuth/API-key credential selection.

This breaks the ability to use CLI provider overrides whenever a user sets their default environment to a custom gateway.

Root Cause

In crates/jcode-base/src/provider/mod.rs:1716-1735, set_credential_mode operates on the currently active provider.

1. During startup, the active provider slot is correctly initialized to the default_provider from the configuration file. For all custom named profiles, this uses the OpenRouter compatibility runtime under the hood.
2. When the user passes --provider openai-api, the initialization sequence attempts to configure the credential mode for the new openai-api route.
3. However, it calls set_credential_mode before properly re-binding the active provider slot to the built-in OpenAI provider.
4. Because the active slot is still pointing to the custom named profile (OpenRouter), set_credential_mode hits the fallback arm at line 1728 and bails out (since only ActiveProvider::Claude and ActiveProvider::OpenAI support credential mode selection).

Workaround

Users currently have to completely omit the --provider openai-api flag and instead pin the route via the model prefix directly: --model openai-api:<model>.

Proposed Fix

Update the provider selection and initialization logic (likely in src/cli/provider_init.rs) to ensure that when an explicit --provider flag is passed, the active provider slot is correctly switched to that requested provider before applying credential routing settings. Alternatively, modify set_credential_mode to accept an explicit provider target rather than relying on self.active_provider().

Start in crates/jcode-base/src/provider/mod.rs around set_credential_mode at lines 1716-1735, then trace the provider initialization path in src/cli/provider_init.rs. Reproduce with a named default_provider and --provider openai-api, and verify that the explicit provider override selects the correct credential route without the OpenRouter error.