feat: add create_mcp_api_integration bootstrap macro and preflight check (closes #24)

[Matts52]

Summary

• Bootstrap macro: Adds create_mcp_api_integration run-operation that generates CREATE API INTEGRATION DDL for external MCP servers. Supports both OAUTH_DYNAMIC_CLIENT (Dynamic Client Registration, recommended) and OAUTH2 (client credentials) auth modes, plus a dry_run=true flag to preview DDL without executing. Requires ACCOUNTADMIN or CREATE INTEGRATION privilege.
• Preflight existence check: snowflake__create_or_replace_cortex_mcp_server now checks via SHOW INTEGRATIONS LIKE + RESULT_SCAN before issuing DDL. If the named API integration is missing, dbt fails immediately with a clear, actionable error message naming the missing object and showing the exact dbt run-operation command to fix it.
• Test fixture: Adds assert_create_mcp_api_integration_ddl run-operation macro that validates DDL output for DCR, OAUTH2, and IF NOT EXISTS modes using dry_run=true — no elevated Snowflake privileges required to run this assertion.
• Documentation: Adds a full cortex_mcp_server section to the README covering the bootstrap workflow, config reference, and complete create_mcp_api_integration parameter table. Updates integration_tests/README.md with the API integration prerequisite and bootstrap command.

Test plan

• dbt run-operation create_mcp_api_integration --args '{integration_name: ..., ..., dry_run: true}' logs expected DDL without executing
• dbt run-operation assert_create_mcp_api_integration_ddl passes — validates DCR, OAUTH2, and IF NOT EXISTS DDL forms (no admin privileges needed)
• On Snowflake with a missing API integration: dbt build on an atlassian_mcp_server model fails with the new preflight error (not a raw Snowflake ORA/SQL error)
• After running create_mcp_api_integration with an admin role: dbt build succeeds end-to-end including atlassian_mcp_server and agent_with_mcp_server

Closes #24

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