Support durable browser pool clear and zero updates

[IlyaasK]

Summary

• preserve explicit --fill-rate 0 on browser-pool updates
• add clear controls for proxy, profile, extensions, and Chrome policy
• serialize SDK-supported empty PATCH sentinels exactly
• reject conflicting set/clear flags before making a request

Why

The merged browser-pool API distinguishes omission from explicit empty and zero values. The CLI previously could not express several durable clear operations and silently omitted a zero fill rate.

Implementation

Update flags track whether numeric values were explicitly supplied. Empty arrays and objects use the SDK's SetExtraFields escape hatch because generated omitzero fields otherwise drop them. Create behavior is unchanged.

Verification

• go test -short -timeout=2m ./...
• go vet ./...
• gofmt -l cmd
• git diff --check
• serialized JSON scenario tests for omission, zero, every clear sentinel, and conflicting flags
• targeted mutation checks: 5/5 durable PATCH mutations killed
• crap4go: validator CRAP 15.1; Update improved from baseline 91.3 to 78.7 through added coverage

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Note

Medium Risk
Changes how pool PATCH payloads are built (including clearing profile/proxy/extensions/policy); mistakes could alter live pool behavior, but scope is CLI-only with strong validation and serialized JSON tests.

Overview
kernel browser-pools update can now express explicit clears and zero values that the API treats differently from omission.

--fill-rate uses an Int64Flag so --fill-rate 0 is sent (pausing auto-fill) instead of being dropped. New --clear-profile, --clear-proxy, --clear-extensions, and --clear-chrome-policy remove durable pool config; empty extensions / chrome_policy on the wire go through SetExtraFields because the SDK omits empty collections. validateBrowserPoolUpdateInput rejects conflicting set/clear flags and negative fill rates before any API call. Empty {} chrome policy now clears policy (replacing the old warning-only behavior). README documents the new flags.

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