Add Action Gateway public APIs

[tgillam-do]

Summary

• add the complete customer-facing Tool Registry REST surface to the public OpenAPI specification
• cover tools, providers, toolkits, tool definitions, toolbelts, OAuth connections, users, and sessions
• model session tool selection, config.preloadTools, invocation policies, returned mcpUrl, and connection metadata
• add generated SDK scopes for read, create, update, and delete operations

Public API coverage

• 4 tool catalog operations under /v2/tools
• 6 Toolbelt operations under /v2/toolbelts
• 5 OAuth connection operations under /v2/connections
• 2 derived user operations under /v2/users
• 3 session operations under /v2/sessions

The spec matches all 20 primary /v2/* REST bindings currently defined by Tool Registry.

Validation

• rebased onto current digitalocean/openapi main and resolved the tag conflict
• source Spectral lint: 0 errors (8 pre-existing warnings)
• bundled Spectral lint: 0 errors (8 pre-existing warnings)
• OpenAPI bundle completed successfully (existing duplicate-schema rename warnings only)
• verified exact path/method parity with tool-registry.swagger.json
• git diff --check