fix(mcp): Address PR review comments - defensive checks and docs

- Validate token_endpoint in OAuth metadata before accessing it
- Check authorization_servers list is non-empty before indexing
- Use provider_matches() (renamed from private _provider_matches) in
  creds_manager for Python 3.13 StrEnum compatibility
- Fill in MCP block documentation with technical explanation and use cases

docs/integrations/block-integrations/mcp/block.md

@@ -1,6 +1,6 @@
 # Mcp Block
 <!-- MANUAL: file_description -->
-_Add a description of this category of blocks._
+Blocks for connecting to and executing tools on MCP (Model Context Protocol) servers.
 <!-- END MANUAL -->
 
 ## MCP Tool
@@ -10,7 +10,9 @@ Connect to any MCP server and execute its tools. Provide a server URL, select a
 
 ### How it works
 <!-- MANUAL: how_it_works -->
-_Add technical explanation here._
+The block uses JSON-RPC 2.0 over HTTP to communicate with MCP servers. When configuring, it sends an `initialize` request followed by `tools/list` to discover available tools and their input schemas. On execution, it calls `tools/call` with the selected tool name and arguments, then extracts text, image, or resource content from the response.
+
+Authentication is handled via OAuth 2.0 when the server requires it. The block supports optional credentials — public servers work without authentication, while protected servers trigger a standard OAuth flow with PKCE. Tokens are automatically refreshed when they expire.
 <!-- END MANUAL -->
 
 ### Inputs
@@ -30,7 +32,9 @@ _Add technical explanation here._
 
 ### Possible use case
 <!-- MANUAL: use_case -->
-_Add practical use case examples here._
+- **Connecting to third-party APIs**: Use an MCP server like Sentry or Linear to query issues, create tickets, or manage projects without building custom integrations.
+- **AI-powered tool execution**: Chain MCP tool calls with AI blocks to let agents dynamically discover and use external tools based on task requirements.
+- **Data retrieval from knowledge bases**: Connect to MCP servers like DeepWiki to search documentation, retrieve code context, or query structured knowledge bases.
 <!-- END MANUAL -->
 
 ---