fix(provider): preserve @cf model identifiers

[Alix-007]

Description

Cloudflare AI model IDs such as @cf/qwen/qwen1.5-0.5b-chat contain slashes that belong to the model path. PicoClaw was treating the first slash as a provider separator, which broke both provider creation and model-reference parsing.

This change treats @... model IDs as bare model identifiers, preserving the full string while still using the default OpenAI-compatible provider path.

Type of Change

• Bug fix
• New feature
• Documentation update
• Refactor

AI Code Generation

• Fully AI-generated
• Mostly AI-generated
• Mostly Human-written

Related Issue

Fixes #1165

Technical Context

• keep @cf/... intact in ExtractProtocol
• keep @cf/... intact in ParseModelRef
• add regression tests for provider creation and model ref parsing

Test Environment

• Hardware: PC
• OS: Linux
• Model/Provider: N/A
• Channels: N/A

Evidence

• go test ./pkg/providers

Checklist

• I reviewed the change.
• I added regression tests.

[Alix-007]

Retained PR is still green: Security Check / Linter / Tests / license/cla all passed on the head. Ready for maintainer review when there is capacity to reconcile the provider fallback ID regression.

[Alix-007]

Retained PR is still green: Security Check / Linter / Tests / license/cla all passed on the fix/issue-1165-atcf-model-id-cleanbase head. Ready for maintainer review when there is capacity to reconcile the provider fallback ID regression.