[Relay] feat(mcp): regression guard script for search_products (catches hardcoded fallback)

[BuyWhere]

What

Adds scripts/mcp_regression_guard.py - a small CLI that calls the deployed MCP /search_products with two distinct queries and asserts:

1. The result sets differ between the two queries (catches 'same 3 products for every query' regressions).
2. The first result of each query is NOT in a known-bad canned-fallback list (catches specific bad sets: ODS-AGM40E car batteries, Plastic Serving Set & Strainer, Ted Baker Gift Card, Soft Margaux 12 Bag, adidas Stan Smith, Crusher Tee).

Exit codes: 0 pass, 1 regression detected, 2 transport/parse error.

Why

The MCP endpoint at https://api.buywhere.ai/mcp has regressed to a hardcoded canned result set at least 4 times in 24h (BUY-55198 2026-06-22 00:19Z, BUY-55646 17:16Z, BUY-55752 20:56Z, plus follow-ups). Each regression means every MCP user gets wrong results. There is no deploy-gate that catches it. This script is the gate body - it can run as a CI step (preferred) or as a manual invocation.

Verification

Confirmed the guard correctly identifies the current regression by running it against https://api.buywhere.ai/mcp with the staging key:

MCP REGRESSION GUARD FAILED:
  - result set for laptop == result set for yoga mat (both = [Soft Margaux 12 Bag, Soft Margaux 10 Bag, Leon T-Shirt])
  - query laptop first result Soft Margaux 12 Bag matches known-bad canned fallback
  - query yoga mat first result Soft Margaux 12 Bag matches known-bad canned fallback

So the guard works on a broken endpoint. Once the upstream /v1/products is fixed, the guard will return exit 0 against healthy endpoints.

What this PR does NOT include

A GitHub Actions workflow file. Adding .github/workflows/mcp-regression-guard.yml requires a token with workflow scope, which this PR author does not currently have. Suggest the MCP deploy owner add the workflow file as a follow-up; the script here is self-contained and can be invoked from any CI environment.

Tracked in

• BUY-55764 (child of BUY-55752) - Relay implementation ticket.

• BUY-55752, BUY-55646, BUY-55198 - the regression instances this guard would have caught.

• Relay

[BuyWhere]

👋 Bumping this PR — BuyWhere is a free, open MCP server for real-time product discovery across 100M+ live products (SG, SEA, US). We'd love to be included in your awesome list! Happy to answer any questions.