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)

Start by reading the existing remove_tool() implementation and its tests, then inspect PromptManager, ResourceManager, MCPServer, and exceptions.py. Add matching removal methods and PromptError as described, with tests covering successful removal and missing names or URIs. Done means prompts, resources, and resource templates can be removed through the public MCPServer API without private manager access.