refactor: centralise provider base URLs — eliminate handler/fetcher duplication

[edelauna]

Problem

Several providers define their base URL independently in both the handler (constructor) and the fetcher, meaning a URL change requires edits in two places. The pattern was introduced with earlier providers and carried forward with each new addition.

Affected providers (handler + fetcher both hardcode the same URL):

Provider	Handler	Fetcher
opencode-go	src/api/providers/opencode-go.ts:35	src/api/providers/fetchers/opencode-go.ts:7
vercel-ai-gateway	src/api/providers/vercel-ai-gateway.ts:31	src/api/providers/fetchers/vercel-ai-gateway.ts:58

Additional providers with hardcoded URLs only in the handler (no fetcher constant to consolidate yet, but still worth noting for consistency):

• fireworks, sambanova, unbound, xai, openai-codex, qwen-code, moonshot

Proposed Fix

Export each provider's base URL constant from packages/types/src/providers/<provider>.ts alongside the existing defaultModelId / defaultModelInfo exports, then import it in both the handler and the fetcher.

Example for opencode-go:

// packages/types/src/providers/opencode-go.ts
export const OPENCODE_GO_BASE_URL = "https://opencode.ai/zen/go/v1"

// src/api/providers/opencode-go.ts
import { ..., OPENCODE_GO_BASE_URL } from "@roo-code/types"
// ...
baseURL: OPENCODE_GO_BASE_URL,

// src/api/providers/fetchers/opencode-go.ts
import { ..., OPENCODE_GO_BASE_URL } from "@roo-code/types"
// remove local const OPENCODE_GO_BASE_URL

Scope

Start with opencode-go and vercel-ai-gateway (both have the handler+fetcher split). Extend to other providers opportunistically.

Surfaced during review of #319.