Summary

Adds support for interfaces and variables with multiple call signatures (function overloads) in kbn-docs-utils API documentation generation. Previously, these declarations were either silently dropped or incorrectly typed, resulting in missing documentation for overloaded functions.

Before / After Example

Given this TypeScript code:

export interface OverloadedFunction {
  (input: string): number;
  (input: number): string;
  (input: boolean): boolean;
}

/** Process the input value using the overloaded function. */
export const overloadedFn: OverloadedFunction = (input: any): any => input;

Before this PR:

The OverloadedFunction interface was exported but its call signatures were lost. The overloadedFn variable was typed as ObjectKind instead of FunctionKind, with no parameter documentation:

{
  "label": "overloadedFn",
  "type": "Object",
  "children": [],
  "signature": []
}
// Call signatures silently dropped! No parameter docs!

After this PR:

The new buildMultipleCallSignaturesDec() handler detects multiple call signatures, extracts parameter docs from the first signature, and produces a proper function declaration:

{
  "label": "overloadedFn",
  "type": "Function",
  "children": [
    {
      "label": "input",
      "type": "CompoundType",
      "description": [],
      "signature": [{ "text": "string" }, " | ", { "text": "number" }, " | ", "boolean"]
    }
  ],
  "signature": [
    "(input: string | number | boolean) => ",
    { "pluginId": "pluginA", "section": "def-public.OverloadedFunction", "text": "OverloadedFunction" }
  ]
}
// ✅ Typed as Function, parameters extracted, signature links to interface

How It Works

Changes

• New buildMultipleCallSignaturesDec() in build_multiple_call_signatures_dec.ts -- handles nodes whose resolved type has multiple call signatures
• Updated buildVariableDec() -- detects when a variable's type has multiple call signatures and delegates to the new handler
• Updated buildFunctionDec() -- extracts parameter source from the first call signature when the node itself has none
• Test fixtures -- adds OverloadedFunction interface and overloadedFn variable to the plugin_a test fixture
• Integration tests -- 4 new test cases verifying interface children, variable type, parameter extraction, and signature linking

Impact

• API declarations for overloaded functions are now correctly typed and documented
• Test plugin apiCount increases from 136 to 145 (new fixture entries)

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