fix(bb-knowledge-base): validate non-integer retrieve maxResults

[osama-rizk]

Fixes #430.

Problem

KnowledgeBase.retrieve normalized maxResults with Math.min(Math.max(options?.maxResults ?? 10, 1), 100) in both index.mock.ts and index.aws.ts. That clamp silently preserves fractional and non-finite values:

• maxResults: 1.5 → the mock slices its result array by 1.5; the AWS runtime sends numberOfResults: 1.5.
• maxResults: NaN → the mock returns an empty set; the AWS runtime sends numberOfResults: NaN.

Bedrock defines numberOfResults as an integer in 1–100, so neither is a valid request — and the mock and AWS runtimes diverge on exactly these inputs (which can arise when the option comes from user input or a calculation).

Fix

A shared normalizeMaxResults helper (new normalize.ts), used by both runtimes:

• undefined → default 10.
• Finite integer → clamped to 1–100 (documented behavior preserved; 0/-5 → 1, 200 → 100).
• Fractional or non-finite (1.5, NaN, ±Infinity) → rejected with KnowledgeBaseErrors.ValidationError, before any local search or Bedrock request.

Also hoisted the blocksError helper (duplicated verbatim in both entries) into errors.ts as the single source, and clarified the RetrieveOptions.maxResults doc.

Tests (red → green)

• normalize.test.ts — default, pass-through, clamp bounds, and rejection of 1.5/NaN/±Infinity.
• index.mock.test.ts — retrieve rejects 1.5/NaN with ValidationError (parity with AWS).
• index.aws.test.ts — retrieve rejects 1.5/NaN with ValidationError and sends no RetrieveCommand.

Verification

• @aws-blocks/bb-knowledge-base suite: 139 pass / 0 fail.
• Conditional-export parity 21/21; check:api ✅ (no public-API change — helper stays internal); npm run lint 0 errors; umbrella-changeset guard ✅.

Changeset

@aws-blocks/bb-knowledge-base patch + @aws-blocks/blocks patch (umbrella re-export).