fix: make source_ids schema provider-compatible

[chrythjin]

Summary

• expose search_tools.source_ids as an optional plain array schema instead of a nullable union
• keep runtime behavior unchanged: callers can still omit source_ids when unused
• add a regression test that asserts the emitted schema type is array

Why

The previous generated schema used type: ["null", "array"]. That is valid JSON Schema, but some MCP clients and provider function-declaration adapters reject nullable union types for tool parameters. Since source_ids is optional, it does not need to include null in the schema type.

This improves compatibility for clients that translate MCP tools into provider function declarations while preserving the existing optional argument behavior.

Verification

• go test ./internal/mcpserver -run TestSearchToolsInputSchema_SourceIDsIsArray
• go test ./...

[rpgeeganage]

Thank you for the contribution 🙇