feat(mcp-actions): Add the ability to configure different plugins for different servers (#33235)

* feat: split MCP actions into per-plugin servers

Add mcpActions.servers config to create multiple MCP server endpoints
scoped by plugin source, with per-server include/exclude filtering.
Add mcpActions.tools for global tool description overrides.

Signed-off-by: benjdlambert <ben@blam.sh>

* feat: namespace tool names, use filter rules for server scoping

- Tool names now use action ID (plugin:name) by default, opt out
  via mcpActions.namespacedToolNames
- Removed pluginSources from server config, use filter.include
  with id glob patterns instead
- Removed tool description overrides (deferred to followup)
- Added server key validation for route safety

Signed-off-by: benjdlambert <ben@blam.sh>

* docs: update README for filter-based server scoping

Signed-off-by: benjdlambert <ben@blam.sh>

* feat: drop SSE routes for split servers

Signed-off-by: benjdlambert <ben@blam.sh>

* fix: handle empty servers config, fix test name

Signed-off-by: benjdlambert <ben@blam.sh>

---------

Signed-off-by: benjdlambert <ben@blam.sh>

.changeset/split-mcp-servers.md

@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-mcp-actions-backend': patch
+---
+
+Added support for splitting MCP actions into multiple servers via `mcpActions.servers` configuration. Each server gets its own endpoint at `/api/mcp-actions/v1/{key}` with actions scoped using include/exclude filter rules. Tool names are now namespaced with the plugin ID by default, configurable via `mcpActions.namespacedToolNames`. When `mcpActions.servers` is not configured, the plugin continues to serve a single server at `/api/mcp-actions/v1`.