docs: note type vs interface for structuredContent

[felixweinberger]

Closes #837. Documents the TypeScript-native workaround for the named interface assignability issue with structuredContent.

Motivation and Context

Issue #837: users hit type errors when using named interfaces for structuredContent. The fix is to use type aliases instead.

Types of changes

• Bug fix
• New feature
• Breaking change
• Documentation update

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Latest commit: 59d0165

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commit: 59d0165

[claude]

LGTM — straightforward docs addition noting a well-known TypeScript gotcha.

Extended reasoning...

Overview

This PR adds a single documentation note to docs/server.md explaining that users should use type aliases instead of interface when defining named types for structuredContent, due to TypeScript's implicit index signature behavior. It closes issue #837.

Security risks

None. This is a pure documentation change with no code modifications.

Level of scrutiny

Minimal scrutiny needed. This is a small, self-contained documentation addition to a markdown file. The technical content is accurate — TypeScript interfaces lack implicit index signatures, so they are not assignable to { [key: string]: unknown }. The workaround (use type or spread) is correct.

Other factors

No outstanding reviewer comments. No changeset needed for a docs-only change. The note is placed in the right location (right after the registerTool example that uses structuredContent). The formatting follows existing conventions in the file (GitHub-flavored admonition blocks).