Live runner: rename internal pixel terminology to pricing/time units (follow-up to #3952, #3942)

Context

After livepeer/go-livepeer#3952 the live runner is priced and metered per second of compute (nanoseconds), and livepeer/go-livepeer#3942 renames the user-facing -pixelsPerUnit flag to -pricingUnitSize. The internal code on the live runner path still uses "pixels" terminology (PixelsPerUnit, inPixels, pixels), which is now a misnomer: the metered quantity is time, not pixels. It only remains for wire compatibility and leftover naming.

Why

Readability and consistency. A developer reading the live runner pricing/payment code sees pixels for what is actually per-second / nanosecond metering, and the names diverge from the pricingUnitSize terminology adopted in livepeer/go-livepeer#3942. This is a naming/cleanup task with no behavior change.

Scope: live-runner-specific (safe to rename)

• ai/runner/live_runner.go: LiveRunnerPriceInfo.PixelsPerUnit and its JSON tag pixels_per_unit -> PricingUnitSize / pricing_unit_size; the converter and convertedPriceInfo locals.
• SDK livepeer-python-gateway live_runner.py: the pixels_per_unit dataclass field -> pricing_unit_size (aligns with livepeer/livepeer-python-gateway#27).
• server/remote_signer.go live runner (lv2v) path: the local pixels variable, the InPixels field / inPixels JSON tag and its "Number of pixels..." comment, and the "pixels" metrics label.

Shared / wire: needs coordination (likely separate)

• server/live_payment.go (calculateFee(inPixels, ...), inPixels fields) and server/live_payment_processor.go (processSegmentFunc(inPixels int64)) are shared with the realtime-AI / transcoding payment paths; rename only if done across all callers.
• net.PriceInfo.pixelsPerUnit (proto field 2) is the wire field shared across transcoding/AI. This is the coordinated, versioned rename already noted as a follow-up in ENG-113 and is out of scope here.

Preconditions (do not start until merged)

• livepeer/go-livepeer#3952 merged (per-second live runner pricing).
• livepeer/go-livepeer#3942 merged (-pricingUnitSize flag).

Related: ENG-113 (pricing flag/term rename).

Wait for go-livepeer#3952 and #3942 to merge, then inspect ai/runner/live_runner.go, live_runner.py in the livepeer-python-gateway SDK, and the live-runner path in server/remote_signer.go. Rename only the listed live-runner-specific identifiers and comments, preserve shared wire fields and payment paths, and confirm the changes are naming-only with no behavior change.