The MCP specification constrains `Annotations.audience` to an array of
`Role` values (`"user"` or `"assistant"`), but the Ruby SDK accepts any
value. The other official SDKs already enforce this (TypeScript, Python,
and PHP).
This follows the existing pattern in `MCP::Icon`, which validates its
constrained `theme` value in the constructor and raises `ArgumentError`.
Added tests in `test/mcp/annotations_test.rb`:
- valid `audience` (`["user"]`, `["assistant"]`, `["user", "assistant"]`,
and an empty array) is accepted
- an unknown role, a mixed valid/invalid array, or a non-array `audience`
raises `ArgumentError`
Existing tests used `["developers"]`, which is not a valid `Role`, and are
updated to `["user"]`.
The full unit suite and RuboCop pass locally.
Strictly speaking this is a breaking change: a caller that currently passes
an invalid `audience` (e.g. `["developers"]`) will now get an
`ArgumentError`. Such values already violate the MCP specification, and
`nil` / valid-role values are unaffected.
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