Add remove_prompt() and remove_resource() for parity with remove_tool()

Description

MCPServer exposes remove_tool(name) (added in #1322) but has no equivalent for prompts or resources. This was part of the original ask in #711 ("removing a tool or resource dynamically"), which was closed when remove_tool() landed — but the resource and prompt sides were never addressed.

Use case

Multi-tenant / multi-instance deployments where the same server image serves different clients. Today users can filter tools per-instance via remove_tool(), but for prompts and resources they're forced to reach into private internals:

# Current workaround — fragile, undocumented
del mcp._prompt_manager._prompts["some_prompt"]
del mcp._resource_manager._resources[str(uri)]

(This is the same pattern @lukehsiao described in https://github.com/modelcontextprotocol/python-sdk/issues/711#issuecomment-2790042502 for tools, before remove_tool() existed.)

Current state

Proposed API

Add these methods, mirroring the existing remove_tool() pattern exactly:

• PromptManager.remove_prompt(name: str) — raises PromptError if not found
• ResourceManager.remove_resource(uri: str) — raises ResourceError if not found
• ResourceManager.remove_template(uri_template: str) — raises ResourceError if not found
• MCPServer.remove_prompt(name) — thin wrapper delegating to the manager
• MCPServer.remove_resource(uri) — thin wrapper delegating to the manager
• MCPServer.remove_resource_template(uri_template) — thin wrapper delegating to the manager
• PromptError exception class in exceptions.py (for symmetry with ToolError and ResourceError)

This is ~15 lines of implementation across 4 source files, plus tests. Purely additive, no breaking changes.

References

• #711 — Original issue requesting dynamic tool/resource removal (closed, only tools were addressed)
• #1322 — PR that added remove_tool() (the pattern this proposal follows)

首先閱讀現有的 remove_tool() 實作及其測試，接著檢查 PromptManager、ResourceManager、MCPServer 和 exceptions.py。依照描述加入相應的移除方法和 PromptError，並加入涵蓋成功移除以及名稱或 URI 不存在情況的測試。當可以透過公開的 MCPServer API 移除 prompts、resources 和 resource templates，且不需要存取私有 manager 時，即表示完成。