fix(mcp): treat empty optionalString args as absent so ctx_search file: "" searches unfiltered

[serhiizghama]

Problem

Fixes #76.

The MCP arg helpers disagree on empty strings. requireString rejects an empty/whitespace-only string, but optionalString (packages/agentctx/src/mcp/tools.ts) only checks the type, so "" (or " ") passes through as a present value.

This has a concrete effect in ctx_search. file comes from optionalString, and the handler branches on file === undefined to decide whether to apply the file filter:

const file = optionalString(args, "file");
const searchLimit = file === undefined ? limit : SEARCH_LIMIT_MAX;
...
if (file !== undefined) {
  const linked = linkedRecordIds(ctx, file); // file = "" → no node matches
  hits = hits.filter((hit) => linked.has(hit.record.id)).slice(0, limit);
}

So ctx_search(query, file: "") is treated as "filter to a file named ''", matches no entity, and returns zero results instead of an unfiltered search. The same gap applies to supersedes (also read via optionalString).

Solution

Return undefined from optionalString for empty/whitespace-only strings, so an optional argument set to "" is equivalent to omitting it. This matches requireString's non-empty rule and the codebase's own nonEmptyString convention in extract/schema.ts. Returning undefined (rather than throwing) is appropriate because file/supersedes are genuinely optional.

No change to the non-empty file/supersedes paths or to requireString.

Testing

Added a regression test in test/mcp/tools.test.ts: with one file-linked record and one unrelated record, ctx_search with file: "" and file: " " now returns the same results as omitting file (both records), not an empty set. Verified it fails on the unfixed helper and passes with the fix.

npm run typecheck   # clean
npm run lint        # clean
npm test            # 298 passed