TW-5383: support detaching a workspace policy via --policy-id ""

[qasim-nylas]

Problem

The CLI cannot detach a policy from a workspace. The Nylas v3 API requires PATCH /v3/workspaces/{id} to carry policy_id as a valid UUID or JSON null — empty string is rejected with policy_id must be a valid UUID or null (api.bad_request). UpdateWorkspaceRequest.PolicyID *string with omitempty serialized &"" as "policy_id":"", so nylas workspace update <id> --policy-id "" always failed and the only workaround was a raw curl call.

This also shows up in nylas agent overview as ⚠ Policy <id> no longer exists — plan maximums apply when a policy is deleted while still attached: the dangling reference could not be cleared from the CLI.

Fix

• Custom MarshalJSON on domain.UpdateWorkspaceRequest: a PolicyID pointing at "" serializes as JSON null (detach); nil still omits the field so rule-only updates don't touch the policy; rule_ids behavior is unchanged — including &[]string{} → "rule_ids":[], which matches the previous omitempty semantics and is required to clear the last rule.
• Agent Studio's workspace PATCH handler gains detach support through the same marshaler (it passes policy_id through as *string).
• nylas workspace update help text and flag description document the empty-string detach.
• docs/commands/agent-policy.md: detach example added; the troubleshooting section no longer suggests swapping in another policy as the only way out.

Tests

• TestUpdateWorkspaceRequest_PolicyIDWireFormat — table test pinning nil → omitted, "" → null, value → string.
• TestUpdateWorkspaceRequest_RulesAndPolicyTogether — pins "rule_ids":[] semantics (clears the last rule) and that both fields serialize together.
• TestHTTPClient_UpdateWorkspace_DetachPolicy — adapter test asserting the PATCH body carries literal null.
• Verified end-to-end against the live API: created a policy, attached it, detached with --policy-id "", confirmed policy_id gone, deleted the policy.
• make ci green.

Related docs

• https://cli.nylas.com/docs/commands/agent-policy-delete — detaching is the fix for the "policy still attached" delete failure described there
• https://cli.nylas.com/docs/commands/agent-policy-list — shows policy↔workspace attachments affected by this change
• https://cli.nylas.com/docs/commands/agent-policy-update — policy lifecycle this command complements
• https://developer.nylas.com/docs/v3/agent-accounts/ — API reference linked from the command help text

Jira: TW-5383

[AaronDDM]

LGTM 👍