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 PATCH payload semantics for browser pools (clear sentinels and fill-rate zero), which can alter live pool behavior when users run update; scope is limited to the CLI update path with new validation and tests.

Overview
kernel browser-pools update now matches the API’s distinction between omitting a field and sending explicit empty/zero values.

Fill rate: --fill-rate uses an Int64Flag so --fill-rate 0 is sent (pauses automatic filling) instead of being dropped when the value was previously treated as “unset.”

Clear flags: New --clear-profile, --clear-proxy, --clear-extensions, and --clear-chrome-policy remove durable pool config (alongside existing --clear-start-url). Conflicting set/clear combinations are rejected in validateBrowserPoolUpdateInput before any API call.

Wire format: Empty extensions and chrome_policy use the SDK SetExtraFields escape hatch so [] and {} survive omitzero encoding. Inline/file {} chrome policy now clears policy instead of warning and doing nothing.

README documents the new flags and behavior. Tests replace empty-policy warning cases with JSON serialization and validation coverage.

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