fix(provider): normalize dotted Anthropic model IDs for API-key paths

[Alix-007]

📝 Description

Normalize dotted Anthropic model IDs like claude-sonnet-4.6 to the API-accepted dashed form claude-sonnet-4-6 in the two API-key paths that were still forwarding the dotted value.

This fixes the API-key anthropic provider path and the native anthropic-messages request builder, and adds regression tests for both.

🗣️ Type of Change

• 🐞 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 📖 Documentation update
• ⚡ Code refactoring (no functional changes, no api changes)

🤖 AI Code Generation

• 🤖 Fully AI-generated (100% AI, 0% Human)
• 🛠️ Mostly AI-generated (AI draft, Human verified/modified)
• 👨‍💻 Mostly Human-written (Human lead, AI assisted or none)

🔗 Related Issue

Fixes #1624

📚 Technical Context (Skip for Docs)

• Reference URL: fix(provider/anthropic): model ID dots not normalized to dashes in API-key and anthropic-messages paths #1624
• Reasoning: pkg/providers/anthropic/provider.go already normalizes dotted Anthropic model IDs for the SDK/OAuth path, but the API-key anthropic branch in CreateProviderFromConfig and the native anthropic-messages request body were still sending dotted IDs. Anthropic rejects those with a 404 and suggests the dashed model ID.

🧪 Test Environment

• Hardware: x86_64 PC
• OS: NewStartOS V4.4.2-ZTE
• Model/Provider: N/A (unit tests only)
• Channels: N/A

📸 Evidence (Optional)

Click to view Logs/Screenshots

$ go test ./pkg/providers/...
ok   github.com/sipeed/picoclaw/pkg/providers
ok   github.com/sipeed/picoclaw/pkg/providers/anthropic
ok   github.com/sipeed/picoclaw/pkg/providers/anthropic_messages
ok   github.com/sipeed/picoclaw/pkg/providers/azure
ok   github.com/sipeed/picoclaw/pkg/providers/common
ok   github.com/sipeed/picoclaw/pkg/providers/openai_compat

$ go test ./pkg/providers/anthropic_messages
ok   github.com/sipeed/picoclaw/pkg/providers/anthropic_messages

go test ./... still fails in unrelated existing/environment-dependent areas (cmd/picoclaw, cmd/picoclaw/internal, cmd/picoclaw/internal/model, pkg/agent, pkg/auth, pkg/migrate/internal, pkg/tools) and was not changed in this PR.

☑️ Checklist

• My code/docs follow the style of this project.
• I have performed a self-review of my own changes.
• I have updated the documentation accordingly.