fix: surface MCP proxy HTTP errors

[RaphaelKhalid]

Summary

• Route the public MCP send-request tool through the existing callAmpersandProxy helper.
• Surface JSON 4xx and 5xx proxy responses as MCP errors.
• Preserve the successful response format and add regression coverage for both paths.

Motivation

The current MCP handler parses any JSON proxy response and returns API call successful, even when the proxy returns a non-2xx status. This makes an upstream failure look like a successful tool invocation to an MCP client or agent. The shared helper already checks response.ok and throws with the status, so this change removes the duplicated request path and aligns MCP behavior with the AI SDK and Mastra adapters.

Related: #40

Testing

• TypeScript type-check passes.
• Targeted ESLint passes for the changed adapter.
• Rebuilt-handler smoke test passes for JSON 502 error classification and unchanged 200 success output.
• git diff --check passes.
• The repository build emits the bundles but exits during declaration generation with the existing API Extractor error Unable to follow symbol for "InferPublicSchema" under the workspace's current dependency resolution. Bun is not available in the environment, so the Bun integration suite could not be run locally.