fix: prevent division by zero in quoteCollateral()

[Kubudak90]

Summary

Fixes #1104

Applies two-layer protection against divide-by-zero in quoteCollateral():

1. Constructor validation (line 138): Change to to reject storeFrontPriceFactor == FACTOR_SCALE
2. Runtime guard (line 1281): Add check before division

Problem

When storeFrontPriceFactor == FACTOR_SCALE (1e18) and liquidationFactor == 0:

• discountFactor = FACTOR_SCALE * (1e18 - 0) / 1e18 = FACTOR_SCALE
• assetPriceDiscounted = assetPrice * (1e18 - 1e18) / 1e18 = 0
• Division by zero in quoteCollateral() line 1287

This permanently disables collateral sales for that market since the config is immutable.

Solution

Two-layer defense:

1. Prevent invalid configuration at deployment time
2. Add runtime guard as defense-in-depth

Checklist

• Bug fix (non-breaking change which fixes an issue)
• New feature
• Breaking change
• Requires documentation update

Testing

• Constructor now correctly rejects storeFrontPriceFactor == FACTOR_SCALE
• Runtime guard prevents division by zero even if constructor check is bypassed
• Existing valid configurations continue to work
• Prevents the division by zero scenario described in quoteCollateral() divides by zero when storeFrontPriceFactor == FACTOR_SCALE and liquidationFactor == 0 #1104