schema: handle typedefs without a parsed signature

[rumblefrog]

Problem

Searching a bundle from the new tree-sitter parser (#30) crashes the frontend:

TypeError: Cannot read properties of null (reading 'arguments')
    at TypeDefinition.searchEntries

The parser sets parsedSignature: null when a typedef or typeset member isn't a function signature, for example typedef Address = int. That matches Option<TypeSignature> in libschema. The TS schema typed the field as always present. The search code before #32 made the same assumption, so it would also have rejected on these bundles. Bundles from the previous parser never contain null, which is why this went unnoticed.

Fix

• parsedSignature is now ITypeSignature | null on ITypeDefinition, IType and the TypeDefinition class.
• Search indexes only the name of a typedef whose signature is null, and skips typeset members whose signature is null.
• @sourcemod-dev/schema is bumped to 0.1.19 so the fix can be published.

Verification

• search_index.test.ts now includes a typedef and a typeset member with parsedSignature: null. With the fix reverted, the test fails with the same TypeError. With the fix, all 8 tests pass.
• tsc --noEmit passes.
• I re-ran the side-by-side comparison against the previous implementation on all 22 bundles on the bundles branch of sourcemod-dev/manifest: zero differences.

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https://claude.ai/code/session_01NLCPZBzDzF1cP5qKEEhEsg

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