test: strengthen marketplace redflags tests with boundary and severity coverage

[YASSERRMD]

Replaces the thin smoke test in internal/marketplace/redflags (which only asserted len(flags) < 4) with rigorous table-driven coverage, matching the rental redflags suite.

Coverage

• TestScanIndividualRules — each rule fires in isolation (non-escrow payment via Western Union/gift card/crypto-only, shipping-via-seller, advance-fee, price-below-market, off-platform pressure), with case-insensitive payment matching and exactly-one-flag assertions.
• TestScanBoundaries — pins exact thresholds (price ratio at 0.3 vs 0.29, zero/empty/negative inputs, inert ShippingMethod) to catch off-by-one regressions.
• TestScanSeverities — locks the severity (1–5) and non-empty message for every code.
• TestScanCleanSubmission — a legitimate listing produces no flags.

Verification

• gofmt clean
• go build ./... passes
• go test ./... passes
• golangci-lint run — 0 issues