fix: use Anthropic SDK provider for anthropic protocol with API key

[echowxsy]

Summary

• Fix anthropic protocol in model_list to use the real Anthropic SDK provider (NewClaudeProviderWithBaseURL) instead of the OpenAI-compatible HTTP provider
• The previous code sent OpenAI-format requests to Anthropic-compatible endpoints, causing 404 errors

Problem

When using model_list with anthropic/ protocol and an API key:

{
  "model_name": "glm-4.7",
  "model": "anthropic/glm-4.7",
  "api_key": "your-key",
  "api_base": "https://api.z.ai/api/anthropic/v1"
}

CreateProviderFromConfig fell into the case "anthropic" branch (line 114) but created an HTTPProvider (OpenAI-compatible), which sent /v1/chat/completions format requests to an Anthropic Messages API endpoint → 404.

Fix

Replace NewHTTPProviderWithMaxTokensFieldAndRequestTimeout with NewClaudeProviderWithBaseURL for the API key path, so it uses the official Anthropic SDK which sends proper /v1/messages format requests.

Testing

Tested on a MaixCAM (RISC-V) device with Zhipu GLM coding plan endpoint (https://api.z.ai/api/anthropic/v1):

Before: 404 Not Found
After:  🦞 2 + 2 = 4 ✅

🤖 Generated with Claude Code

[nikolasdehor]

Good fix for the model_list path — this correctly routes Anthropic API key configs through NewClaudeProviderWithBaseURL instead of the OpenAI-compatible HTTP provider.

Notes:

1. Complementary to PR #444: This PR fixes factory_provider.go (CreateProviderFromConfig, for model_list configs), while PR #444 fixes factory.go (resolveProviderSelection / CreateProvider, for single-provider configs). Both should be merged for complete coverage.

2. Dropped parameters: The old code passed cfg.Proxy, cfg.MaxTokensField, and cfg.RequestTimeout to the HTTP provider. NewClaudeProviderWithBaseURL does not accept these — consider whether this is a regression for users with proxy settings configured.

3. Empty apiBase: When cfg.APIBase is empty, the old code set a default https://api.anthropic.com/v1. Verify that NewClaudeProviderWithBaseURL handles empty apiBase correctly (likely via Anthropic SDK defaults).

4. Tests: Please add at least one test case for CreateProviderFromConfig with anthropic protocol and API key.

[echowxsy]

Thanks for the thorough review! Addressing each point:

Complementary to PR #444: Agreed — this PR fixes CreateProviderFromConfig (model_list path) while #444 fixes CreateProvider (single-provider path). Both should be merged for complete coverage.

Dropped parameters (Proxy, MaxTokensField, RequestTimeout): This is intentional and consistent with the existing OAuth/token auth path (lines 115-121), which also uses ClaudeProvider without these parameters. The Anthropic SDK manages HTTP internally via anthropic-sdk-go and doesn't expose proxy/timeout at the constructor level. If proxy support for Anthropic protocol becomes needed, it would require passing option.WithHTTPClient() with a custom transport — that could be a separate enhancement PR.

Empty apiBase: Verified — normalizeBaseURL("") returns defaultBaseURL ("https://api.anthropic.com"), so empty api_base correctly falls back to the official Anthropic API endpoint.

Tests: Added in 3e1e2b7:

• TestCreateProviderFromConfig_Anthropic — updated to verify *ClaudeProvider type (not *HTTPProvider)
• TestCreateProviderFromConfig_AnthropicWithCustomAPIBase — tests custom api_base (e.g., Zhipu coding plan endpoint)
• TestCreateProviderFromConfig_AnthropicMissingAPIKey — verifies proper error message

All provider tests pass: go test -tags stdjson ./pkg/providers/ ✅