fix(model): preserve explicit null structuredContent in CallToolResult

[JMLX42]

Motivation and Context

The MCP 2026-07-28 specification, Tools > Structured Content says:

Structured content is returned as a JSON value in the structuredContent field of a result. This can be any JSON value (object, array, string, number, boolean, or null) that conforms to the tool's outputSchema if one is defined.

For backwards compatibility, a tool that returns structured content SHOULD also return the serialized JSON in a TextContent block.

The 2026-07-28 schema types CallToolResult.structuredContent as unknown ("This can be any JSON value (object, array, string, number, boolean, or null)").

So null is a legal structured result, and the TextContent copy is only a SHOULD. Today CallToolResult's custom Deserialize reads structured_content as a plain Option<Value>, which decodes a present null as None, the same as an absent field:

1. {"resultType":"complete","content":[],"structuredContent":null} decodes with structured_content == None: the explicit null is lost.
2. {"resultType":"complete","structuredContent":null} fails to decode as CallToolResult ("expected at least one known CallToolResult field"), so inside ServerResult it falls through to CustomResult.

We hit this while proxying tool results whose structured value is legitimately null.

Fix

• The deserialization helper decodes a present structuredContent as Some(value) (#[serde(default, deserialize_with = "deserialize_present_value")]). An explicit null becomes Some(Value::Null) and counts as a known field in the existing guard.
• The public type stays Option<Value>. Serialization is unchanged: skip_serializing_if = "Option::is_none" already omits only None, so Some(Value::Null) serializes as "structuredContent": null.
• The field's doc comment (and the committed message schema description) now says "any JSON value, including null" instead of "JSON object".

How Has This Been Tested?

New tests, all failing on main except the two guards:

• test_structured_output.rs: explicit null with content is preserved; explicit null without content decodes; an absent field stays None; Some(Value::Null) round-trips as "structuredContent": null while None omits the field.
• test_deserialization.rs (untagged_server_result): both payloads above, plus {"structuredContent": null}, deserialize as ServerResult::CallToolResult with Some(Value::Null); an object with only unknown null fields still falls through to CustomResult (the fix: prevent CallToolResult from shadowing CustomResult in untagged enum deserialization #771 invariant).

cargo test --all-features, cargo +nightly fmt --all -- --check, and both clippy invocations from CI pass locally.

Breaking Changes

None. Absent structuredContent still decodes as None, and the public field type is unchanged. The only behavior change is that a present null now decodes as Some(Value::Null) instead of None, and the second payload above now decodes as CallToolResult instead of failing.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

Related but distinct: #750, #919, #1046, #1094.

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🤖 Written by Claude Code (2.1.280) using model claude-opus-5-5

[DaleSeo]

Thanks for the fix, @JMLX42!