[Bug]: Read-only MCP search tools fail with missing provider errors

What happened?

A read-only credential created in Workspace Settings → Integrations → MCP Server exposes keyword_search and semantic_search, but both fail before returning documents.

Steps to reproduce

1. Connect an MCP client to a workspace using a read-only credential.
2. Call keyword_search with {"query":"SLA"} or {"query":"AFFiNE"}.
3. Call semantic_search with {"query":"SLA"}.

Actual result

• keyword_search: Mcp error -32001: Error executing tool: Search provider not found.
• semantic_search: Mcp error -32001: Error executing tool: No copilot provider available: gemini-embedding-001.

Expected result

Return search matches or an empty result. If a search or embedding provider must be configured, show that prerequisite and an actionable error.

Environment

• MCP client: Codex on Windows
• AFFiNE: custom-domain workspace; server version unconfirmed
• Credential: read-only
• Backend configuration and logs unavailable

Related: #15038 reports a different keyword_search error on no-result queries; #15424 reports a similar Gemini provider error outside the MCP flow.

Drafted with AI assistance from actual MCP tool responses at the user's request. No private URL, workspace ID, or credential is included.