fix: preserve imported providers and use provider model catalog

[k9ight000]

What changed

• Preserve existing CodexPlusPlus relay profiles when importing providers from CC Switch; append only new provider identities.
• Fetch the active provider's model catalog from its /v1/models endpoint.
• Stop mixing provider models with the configured model, local model catalog, or frontend custom-model fallback.
• Add regression coverage for provider-only model catalogs and non-overwriting imports.

Validation

• cargo test -p codex-plus-core --test model_catalog — 7 passed.
• cargo test -p codex-plus-core --lib ccs_import::tests — 3 passed.
• cargo test -p codex-plus-core --test cdp_bridge — 77 passed.
• node --check assets/inject/renderer-inject.js passed.
• The serial core test run passed 318 tests; one unrelated Windows symlink test requires the SeCreateSymbolicLinkPrivilege privilege in this environment.

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