Empty cacheScope on tools/list drops every tool

Describe the bug

Some hosted MCP servers send cacheScope: "" on tools/list and resources/read results. SEP-2549 only allows "public", "private", or omitting the field, so serde rejects the empty string. Because ServerResult is an untagged enum, that failure does not stay on ListToolsResult. The payload falls through to CustomResult, and the client ends up with no tools.

Negative ttlMs is already normalized. Empty cacheScope is not.

To Reproduce

Deserialize a valid tools/list result that includes an empty cache scope:

{
  "tools": [{ "name": "search", "inputSchema": { "type": "object" } }],
  "ttlMs": 0,
  "cacheScope": ""
}

On current main this does not parse as ListToolsResult.

Expected behavior

Treat an exact empty cacheScope the same as an omitted field (None). Unknown values such as "shared" or " " should still fail.

Additional context

I hit this against a hosted server that emits cacheScope: "" on an otherwise valid tools/list body. The tools were present on the wire and disappeared after decode.

Start with the serde deserialization for cacheScope in the tools/list and resources/read result types, then reproduce the provided JSON as a focused test. Done means an exact empty string is treated like an omitted field, tools remain available, and values such as "shared" or " " still fail.