## Motivation and Context
MCP 2025-11-25 spec defines a `_meta` general field on many types,
but the Ruby SDK only exposed it on `Tool` and `Prompt`.
To attach metadata (e.g. MCP Apps CSP hints on resource contents),
consumers had to subclass existing classes and override `to_h`.
Add a `meta:` keyword argument and `_meta` serialization to:
- `Resource::Contents`, `Resource::TextContents`, `Resource::BlobContents`
- `Resource`, `ResourceTemplate`
- `Content::Text`, `Content::Image`, `Content::Audio`, `Content::EmbeddedResource`
- `Tool::Response` (`CallToolResult`)
- `Prompt::Result` (`GetPromptResult`)
This aligns the Ruby SDK with the Python and TypeScript SDKs, which both expose `_meta` on these types.
Resolves#308.
## How Has This Been Tested?
Added unit tests for each updated class covering two patterns: `_meta` present
(asserts the value is reflected in `to_h[:_meta]`) and `_meta` omitted
(asserts no `:_meta` key in `to_h`, i.e., `.compact` behavior).
All existing tests continue to pass, and `rake rubocop` is clean.
## Breaking Changes
None. All new `meta:` parameters are optional keyword arguments defaulting to `nil`.
The `to_h` output stays backward-compatible because `nil` values are removed by the existing `.compact` call,
so existing callers require no changes.
Due to overly aggressive detection by RuboCop Minitest, the distinction between `[]` and `{}` was uniformly replaced with `assert_empty`,
making it impossible to differentiate between them.
As a result of https://github.com/rubocop/rubocop-minitest/issues/344, where the `Minitest/AssertEmptyLiteral` cop was disabled,
the return value has been expressed using explicit literal forms.
Follow-up to https://github.com/modelcontextprotocol/ruby-sdk/pull/147.
According to the specification schema, `content` is not optional and therefore cannot be omitted:
```typescript
interface CallToolResult {
_meta?: { [key: string]: unknown };
content: ContentBlock[];
isError?: boolean;
structuredContent?: { [key: string]: unknown };
[key: string]: unknown;
}
```
https://modelcontextprotocol.io/specification/2025-06-18/schema#calltoolresult
Instead of `nil`, an empty array is set as the default value.
There may be a better value for `content`, but at the very least it should not be missing.
Both the combination of positional argument and naming does not feel
like Ruby.
This change makes `is_error` argument a keyword argument called `error`.
Thanks to this, the usage is more self-explanatory:
```ruby
response = Response.new([], error: true)
```
