Tool and resource conventions: the tool provider extension point, structured results, the schema resource and read-only tools #2504

Part of #2423 (F-MCP). The tool and resource conventions every capability epic builds its tools on (MCP-02, MCP-03, UX-03), with the first read-only tools for content types and change sets. Tool providers are an extension point (F-EXT #2429, EXT-01), so built-in tools register the same way a plugin's will. Agents authenticate since #2485 and every call is logged since #2488, so tools can now land.

Scope

• Core (FluentCMS.Core.Tools):
  • [ExtensionPoint] IToolProvider: the tools of one capability. A tool has a snake_case name, a title, a description written for an LLM reader, a JSON Schema for its input with a description on every argument, a read-only flag and a handler.
  • [ExtensionPoint] IResourceProvider: resources by URI or URI template (MCP-03), with a name, a description, a media type and a handler.
  • ToolResult: a structured JSON value, or the errors of a Result, each with a code and a message that says what to change (UX-03).
  • ToolArgumentsReader: reads required and optional arguments and reports a missing, malformed or unknown argument by name, so an agent's typo comes back as an error instead of being ignored. It works as FieldSettingsReader does for field settings.
  • ToolProviderConformanceTests, which every provider passes, built-in or plugin: unique snake_case names, a described input schema that refuses unknown arguments, and a description for every tool and argument.
• Mcp maps every registered tool and resource onto the SDK on both transports.
  • Tools carry the read-only annotation.
  • A success returns the value as structured content and as JSON text.
  • A failure is a tool error (isError) with {"errors": [{"code", "message"}]} in both forms, never a protocol error.
• Built-in tools (FluentCMS.Core.Builtin):
  • list_content_types
  • get_content_type (slug, optional version)
  • get_schema_document (optional slug; the site's document without one, SCH-06, Add the schema document with field descriptions and examples #2473)
  • get_change_set (id)
  • get_change_set_diff (id, CHG-03)
• Built-in resources: fluentcms://schema, the site's schema document, and the template fluentcms://schema/{slug}, one type's.
• CONTRIBUTING.md: the conventions, under "Extension points".

Out of scope

• Write tools for content types and change sets, including session grouping: the next slice. Agents never approve or publish (MCP-06), so no such tool exists.
• Bulk operations with a change set id and progress, and the MCP-01 coverage test (MCP-08, MCP-01).
• Tools and resources for entries and entities: the F-CNT (F-CNT: Entries with versions, states and entity references #2416) and F-ENT (F-ENT: Entity registry with sources, validity and blast radius #2417) lanes add them on these conventions.
• Rule hints in descriptions and the active-rules resource: after F-RUL (F-RUL: Rule book as versioned content #2419).
• Scope checks on reads: with the agent scopes, in the credentials lane of F-MCP: MCP server, agent credentials, scopes and caps #2423.

Acceptance

• Over HTTP and stdio, tools/list returns the five tools, each with a description, an input schema and the read-only annotation.
• list_content_types lists each type's id, slug, name and version. get_content_type returns the entity flag, the fields and the definition at the current version or at a named one.
• get_schema_document returns the same document as ISchemaDocuments, for the site or for one type.
• get_change_set returns a stored set with its operations and state, and get_change_set_diff its field-level diff.
• An unknown slug, version or change set is a tool error with the service's code and a message that says how to find a valid one.
• A missing, malformed or unknown argument is a tool error that names the argument and the form it takes, and the service is not called.
• resources/list and resources/templates/list show the schema resources. Reading one returns the JSON document, and an unknown slug is an error.
• Every built-in provider passes the conformance tests, and two tools with one name are refused at startup.
• Everything above passes on SQLite and PostgreSQL, dotnet build -warnaserror passes, and ProjectReferenceRulesTests pass: Core gains no package, and Mcp references only Core.