stateless: tools/list leaks x-mcp-file to clients that did not declare fileInputs #1515

Problem

SEP-2356 says a client that did not declare the fileInputs capability MUST NOT see the x-mcp-file keyword in tool input schemas. The session wire strips it (server/dispatch.go:573, stripFileInputsFromTools). The stateless wire does not. server/stateless/handlers.go:19-31 returns d.Backend.Tools() unfiltered, and its comment says per-request stripping "lands when the example fixture starts exercising the path".

Reproduced on main at 88052be3. A stateless client with no fileInputs declaration calls tools/list against a server holding a tool with an x-mcp-file property, and gets:

{"name":"upload_report","inputSchema":{"properties":{"report":{"format":"uri","type":"string","x-mcp-file":{"accept":["application/pdf"]}}},"type":"object"}}

The same call on the session wire returns the property without x-mcp-file.

Why it matters now

experimental/ext/agents (#1393) must return tool definitions in agents/get that match the caller's tools/list field for field (agents-wg#22). It reads the per-request declaration via ctx.ClientCaps() and strips correctly on both wires, so on the stateless wire it now disagrees with tools/list for exactly this case. The agents parity test covers every other combination.

Work

• Strip x-mcp-file in the stateless tools/list when the per-request _meta carries no fileInputs capability (core.RequestMetaFromContext, per the capability rule in CLAUDE.md).
• Test on the 2026-07-28 wire, with and without the declaration.
• Add the stateless no-fileInputs case to the agents parity test (experimental/ext/agents/stateless_test.go).