Config errors should only surface details in STDIO mode #781

Problem

Errors like ConfigurationError and LLMProviderError (invalid keys, missing providers, conflicting config) currently show detailed configuration guidance to all users regardless of transport. In the hosted/Cloudflare deployment (mcp.sentry.dev), users have no ability to fix server-side configuration — these messages are confusing and unhelpful. They should see a generic "feature unavailable" message, while the detailed config error is logged server-side for operators.

In STDIO mode, the user controls their own environment, so detailed config messages are appropriate and should be preserved.

Proposed Solution

1. Add a field to ServerContext that indicates whether the user can fix config issues (e.g., transport: "stdio" | "http" or similar)
2. Set it in the STDIO entry point (packages/mcp-server/src/index.ts) and Cloudflare entry point (packages/mcp-cloudflare/src/server/lib/mcp-handler.ts)
3. Update formatErrorForUser() in packages/mcp-core/src/internal/error-handling.ts to accept this context:
   • For ConfigurationError and LLMProviderError in HTTP mode: return generic "Feature Unavailable" message, log details server-side
   • For ConfigurationError and LLMProviderError in STDIO mode: preserve current detailed guidance
   • For APICallError 4xx in HTTP mode: same generic treatment
4. Pass the context from server.ts tool error catch block into formatErrorForUser()

Key Files

• packages/mcp-core/src/types.ts — ServerContext (needs transport/environment awareness)
• packages/mcp-core/src/internal/error-handling.ts — formatErrorForUser() (needs conditional formatting)
• packages/mcp-core/src/server.ts — tool error catch block (needs to pass context)
• packages/mcp-server/src/index.ts — STDIO entry point
• packages/mcp-cloudflare/src/server/lib/mcp-handler.ts — Cloudflare entry point

Acceptance Criteria

• STDIO users see detailed config error messages (current behavior preserved)
• HTTP/Cloudflare users see generic "feature unavailable" message for config/provider errors
• Detailed error info is logged server-side for operators in HTTP mode
• Tests cover both transport modes

Related to #779