Prebid Mobile: makes creative size transformation based on pxRatio.

[mdanylov-sigma]

Description

Fixes #4501

Prebid Server was not converting device.w/device.h from physical pixels to device-independent pixels (DIPs) when generating the list of candidate sizes for interstitial ads. According to the OpenRTB spec, device.w/device.h represent physical screen pixels, while banner.format sizes are expected in DIPs. On high-density screens this caused standard mobile interstitial sizes (e.g. 320x480) to be absent from the generated format list.

This change adds opt-in pxratio conversion controlled by a new field ext.prebid.sdk.usepxratio. When set to true, device.w/device.h are divided by device.pxratio before being used to calculate the interstitial size bounds. Conversion is only applied when falling back to device dimensions — explicit banner.format sizes are left untouched.

Changes

• Added usePxRatio (Boolean, JSON: usepxratio) field to ExtRequestPrebidSdk; added explicit of() factory overloads to preserve backward compatibility with existing callers
• Added usePxRatio(BidRequest) helper to read the flag from ext.prebid.sdk
• Added deviceSizeToDips(Integer, BigDecimal) helper: divides by pxratio using doubleValue(), rounds to nearest integer, clamps to minimum 1; returns size unchanged when size or pxratio is null or non-positive
• Applied conversion in processInterstitialImp() only in the device-size fallback branch

Testing

• processShouldReturnBidRequestUpdatedWithInterstitialFormatsUsingDeviceSizeInDipsWhenFormatIsEmpty — device 1080×1920 @ pxratio 3 → expects 320×480 in results
• processShouldReturnBidRequestUpdatedWithInterstitialFormatsUsingDeviceSizeInDipsWhenFormatIsOneToOne — same, with 1×1 sentinel format
• processShouldNotConvertExplicitFormatSizeUsingDevicePxratio — explicit banner format is not converted
• processShouldKeepCurrentDeviceSizeBehaviorWhenUsePxRatioIsAbsent — no flag, no conversion (backward compatibility)
• processShouldKeepCurrentDeviceSizeBehaviorWhenUsePxRatioIsTrueAndDevicePxRatioIsAbsent — flag set but pxratio missing, sizes unchanged