Prefer global provider credentials / 优先使用全局模型服务凭据

[SivanCola]

Summary

• prefer user-global provider entries over same-named project provider entries, while preserving shadowed project providers when project configs are saved
• use global-first credential resolution for provider keys, with existing process env first and project/home .env as fallbacks
• keep desktop Settings credential display side-effect free so opening Settings does not mutate runtime environment variables
• avoid canonicalizing custom provider aliases unless the matching provider is an official endpoint

Related

• Related to Fix desktop provider resolution across project configs #4516, but this PR focuses on the global-first precedence contract for provider config and credentials.

Testing

• go test ./internal/config
• go test . from desktop/
• go test ./...

[SivanCola]

Resolved the merge conflict by rebasing this PR onto the latest main-v2.

Verification:

• go test ./internal/config
• go test . from desktop/

Current head: ca0ee6e93e55a4ffb6d97bcbcf0c7acc7607f968

[chatgpt-codex-connector]

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