feat(llm): add Alibaba Cloud Coding Plan support (#1388)

[yykaue]

Description

Adds first-class Alibaba Cloud Coding Plan support so subscribers can configure OCR with dedicated endpoints and API keys.

• Add dashscope-codingplan and dashscope-codingplan-intl presets for China and international endpoints.
• Reuse the existing OpenAI Chat Completions client for chat and tool calling.
• Support dedicated environment variables and model selection in CLI configuration and the VS Code extension.
• Document authentication, endpoint configuration, connectivity checks, and differences from Token Plan in all five documentation languages.
• Clarify that OCR's token usage does not represent the remaining Coding Plan quota.

Type of Change

• New feature (non-breaking change that adds functionality)
• Documentation update

How Has This Been Tested?

• make check
• make test
• make coverage — 91.9% overall coverage
• make build
• VS Code extension tests — 12 suites and 107 tests passed
• Extension TypeScript checking and linting of changed TypeScript files

Added tests covering provider presets, credential resolution and isolation, model validation, tool-call round trips, ocr llm test, and VS Code configuration persistence.

Connectivity and tool-calling tests use local HTTP mocks. Live connectivity with a Coding Plan subscription has not been verified.

Related Issues

Closes #1388

[CLAassistant]

All committers have signed the CLA.

[github-actions]

🔍 OpenCodeReview found 1 issue(s) in this PR.

• ✅ Successfully posted inline: 1 comment(s)

[github-actions]

Missing required test coverage: Per the project rules, every new provider MUST have a corresponding TestLookupProvider_<Name>Details test in internal/llm/providers_test.go that verifies Protocol, BaseURL, EnvVar, and Models. Neither dashscope-codingplan nor dashscope-codingplan-intl has such a test. The TestListProviders_Order list was updated, but detail-level regression guards are absent.

Add a test similar to the existing TestLookupProvider_MiniMaxDetails pattern, e.g.:

func TestLookupProvider_DashScopeCodingPlanDetails(t *testing.T) {
    tests := []struct {
        name, wantURL, wantEnvVar string
        wantProtocol              Protocol
        wantModels                []string
    }{
        {
            name:         "dashscope-codingplan",
            wantURL:      "https://coding.dashscope.aliyuncs.com/v1",
            wantEnvVar:   "DASHSCOPE_CODINGPLAN_KEY",
            wantProtocol: ProtocolOpenAIChatCompletions,
            wantModels:   []string{"qwen3.7-plus", "qwen3.6-plus", "kimi-k2.5", "glm-5", "MiniMax-M2.5", "qwen3.5-plus", "qwen3-max-2026-01-23", "qwen3-coder-next", "qwen3-coder-plus", "glm-4.7"},
        },
        {
            name:         "dashscope-codingplan-intl",
            wantURL:      "https://coding-intl.dashscope.aliyuncs.com/v1",
            wantEnvVar:   "DASHSCOPE_CODINGPLAN_INTL_KEY",
            wantProtocol: ProtocolOpenAIChatCompletions,
            wantModels:   []string{"qwen3.7-plus", "qwen3.6-plus", "kimi-k2.5", "glm-5", "MiniMax-M2.5", "qwen3.5-plus", "qwen3-max-2026-01-23", "qwen3-coder-next", "qwen3-coder-plus", "glm-4.7"},
        },
    }
    // ... assert each field
}

[chaojixinren]

LGTM

[lizhengfeng101]

Heads up — #1031 just landed on main and moved the VS Code webview and shared types into a new extensions/frontend/ package, so please rebase onto the latest main and relocate any changes under extensions/vscode/src/webview/ or extensions/vscode/src/shared/ to extensions/frontend/src/.