Skip resize + chromium restart when display already matches

[masnwilliams]

Summary

VM-side companion to the API-side short-circuit. Both PatchDisplay and the multipart ChromiumConfigure (when display is the only thing in the request) currently proceed through the full xrandr/xvfb resize plus a chromium restart even when the requested width/height/refresh-rate already match the current display state. This change detects the no-op and returns success immediately — skipping the idle check, recording stop, resize, and restart.

Sister PR on the API side: kernel/kernel#2233 (skips the configure call entirely when the request matches the pool image default). This PR catches the same case from the VM side, plus any caller that passes a non-default-but-already-current viewport.

Changes

• Extract resolveDisplayParams so PatchDisplay and chromiumPrepareDisplay share one definition of "the request changes nothing."
• PatchDisplay returns 200 immediately when changed == false.
• chromiumPrepareDisplay returns a nil plan, which the caller already handles as skip-apply.

Test plan

• go test ./cmd/api/api/ — new TestResolveDisplayParams covers matching dims with and without refresh-rate, mismatched width, refresh-rate-only mismatch, partial body, and nil body
• Manual: deploy to a metro, send PatchDisplay {width:1920,height:1080} to an instance launched at 1920x1080, confirm no chromium restart in supervisor logs and no xrandr/xvfb activity

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Note

Low Risk
Targeted early-return on display APIs with unit tests; avoids disruptive side effects on no-op requests rather than changing resize semantics when values differ.

Overview
Adds a VM-side no-op path when display resize requests already match the current width, height, and refresh rate, so callers avoid idle checks, recording stops, xrandr/Xvfb work, and Chromium restarts.

resolveDisplayParams centralizes merging the request with current display state and sets a changed flag when any of those fields would differ. PatchDisplay returns 200 immediately when nothing would change; chromiumPrepareDisplay returns a nil plan so configure skips display apply (existing behavior for nil plans).

TestResolveDisplayParams covers matching/mismatch cases, partial body fields, and nil body.

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