[Code Quality] refactor: introduce named type LLMProvider string to replace bare string threading through ~8 functions

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Description

The LLMProvider concept is threaded through approximately 8 functions as a bare string, including secret selection logic and gateway routing decisions. Using a named type improves type safety, enables exhaustive switch checking, and documents intent at call sites.

Current State

// Currently — bare string driving secret selection and gateway routing
func selectSecret(provider string) string { ... }
func routeToGateway(provider string) url.URL { ... }
// EngineConfig.LLMProvider is string

Suggested Changes

1. Add type LLMProvider string with its constants (likely in pkg/types or pkg/cli)
2. Update EngineConfig.LLMProvider field from string to LLMProvider
3. Update the ~8 functions that accept/return provider values
4. Add a String() string method for logging compatibility

Files Affected

• Likely pkg/cli/ or pkg/types/ for the type definition
• ~8 functions across compiler/gateway/secret-selection code (exact files to be identified at implementation time via grep -r "LLMProvider" --include=*.go)

Success Criteria

• type LLMProvider string defined with named constants
• EngineConfig.LLMProvider is LLMProvider not string
• No bare string arguments for provider values at key routing/selection call sites
• go build ./... and make test-unit pass

Source

Extracted from discussion #47781 — Typist Go Type Consistency Analysis

Priority

Medium — Improves type safety at gateway routing and secret selection boundaries

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• expires on Jul 28, 2026, 11:13 AM UTC-08:00