Support swapping proxies

[sjmiller609]

Note

Introduces proxy updates for running browser sessions via the CLI.

• Adds Update to BrowsersService and implements BrowsersCmd.Update to set ProxyID or clear it (mutually exclusive --proxy-id / --clear-proxy)
• New browsers update cobra command with validation and optional --output json
• Extends tests’ FakeBrowsersService with Update support
• Bumps dependency github.com/kernel/kernel-go-sdk to v0.26.0

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