fix(mcpserver): advertise capabilities only for registered primitives

[Zelys-DFKH]

Summary

• MCPServer.__init__ unconditionally passes non-None list handlers to
  the lowlevel Server, causing get_capabilities() to always advertise
  tools, resources, and prompts, even when none are registered.
• Adds an optional capability_filter parameter to the lowlevel Server
  that post-processes the computed ServerCapabilities.
• MCPServer passes a filter that suppresses each entry when the
  corresponding manager is empty at capability-computation time.

Motivation and Context

Per the MCP schema spec and lifecycle docs, a ServerCapabilities entry should appear only when the server actually offers that primitive. Currently, an empty MCPServer advertises all three, and a server with only one tool still advertises resources and prompts it doesn't have.

The filter runs when create_initialization_options() is called at the start of run(), after all @mcp.tool() / @mcp.resource() / @mcp.prompt() decorators have been applied. Every transport path (stdio, SSE, streamable HTTP) calls server.create_initialization_options() → get_capabilities(), so the filter applies without any per-transport changes.

How Has This Been Tested?

Six new tests in tests/server/mcpserver/test_server.py cover: empty server, tool-only, resource-only, resource-template-only, prompt-only, and all-registered cases.

uv run --frozen pytest tests/server/mcpserver/test_server.py -v

Full suite: ./scripts/test — 100% branch coverage.

Breaking Changes

None. Servers that previously over-advertised will now send correct capability sets.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Closes #2473