Plugin configs: identify and wire unused config fields

[N0153128]

Re-scan of all plugin configs against their processor and validator call sites reveals several
fields that are defined and have getters but are never read by anything in the plugin pipeline.

ImageConfig — logoSizeLimit has a getter but is never checked. Logo files have no size
validation gate.

AudioConfig — maxSampleRates and maxChannels are never read by AudioValidator. Sample
rate and channel count are detected but not validated against configured limits. bitrateFallback
and samplingRateFallback are never read — the processor uses values from context directly.
outputBitrate, outputSampleRate, and outputChannels are never read — the processor pulls
these from context populated by MediaUtils rather than from config.

VideoConfig — maxVideoWidth, maxVideoHeight, and maxVideoFrameRate are never read by
VideoValidator. Dimension and frame rate checks are entirely absent from video validation.
outputVideoBitrate, outputSampleRate, outputChannels, and outputFrameRate are never
read — same issue as audio, processor uses context values instead of config.

Each unused field needs a decision: wire it into the appropriate validator or processor, or
remove it if the behaviour it was meant to drive is covered elsewhere.

Acceptance Criteria

• logoSizeLimit wired into a logo size check in ImageValidator or ImageProcessor
• maxSampleRates wired into AudioValidator
• maxChannels wired into AudioValidator
• AudioConfig fallback and output fields either wired into AudioProcessor or removed if redundant
• maxVideoWidth, maxVideoHeight, maxVideoFrameRate wired into VideoValidator
• VideoConfig output fields either wired into VideoProcessor or removed if redundant