fetch: call_tool and get_prompt ignore the requested name #4988

Problem

call_tool in server.py never checks the tool name, so call_tool("nope", {"url": ...}) fetches the URL. get_prompt likewise never checks the prompt name and serves the fetch prompt for any name.

Where it is pinned

src/fetch/tests/test_protocol.py: test_call_tool_never_checks_the_tool_name, test_unknown_tool_name_still_validates_through_pydantic, test_get_prompt_never_checks_the_prompt_name, added in #4971 (Wave 1 of #4857). Each of these tests carries a KNOWN BUG marker: it asserts the current, wrong behavior, so the fix has to change it.

Expected

An unknown tool name returns an "Unknown tool" error, and an unknown prompt name returns an error rather than the fetch prompt.

Done when

• The behavior above is fixed, in the legacy-era (2025-11-25) server on SDK 1.x.
• The pinning tests assert the correct behavior, and their KNOWN BUG markers are gone.
• The per-file 90% coverage gate still passes.