agent host: collapse the client-state parameters of resolveCustomizationEnablement

Follow-up cleanup from the scoped-enablement work (#330566, #330715).

## Problem

`resolveCustomizationEnablement` in `src/vs/platform/agentHost/node/shared/customizationEnablementGate.ts` now takes six parameters, the last three of which are all optional and all different views of the same client state:

```ts
export function resolveCustomizationEnablement(
service: IAgentHostCustomizationEnablementService,
session: URI,
customizations: readonly Customization[],
clientChildEnablement?: ReadonlyMap>>,
clientPlugins?: ReadonlyMap,
mcpServerOwners?: ReadonlyMap,
): IResolvedCustomizationEnablement
```

There are 14 call sites across 5 files, and because every extra argument is optional, **omitting one is silent**. That is not hypothetical:

- Codex never supplied the ownership map to its `McpCustomizationController`, so a promoted plugin-owned MCP server keyed as `mcpServers#` instead of `#mcp=` and forgot its global/workspace decision across sessions. Fixed in #330715, but only after it was found by hand.
- Auditing whether a given omission is a bug is genuinely hard. Codex's two `resolveCustomizationEnablement` call sites still omit `mcpServerOwners`, and that is *fine* — they only pass plugin candidates, so the nested branch derives ownership from the plugin in hand. But establishing that took several minutes of tracing. A reviewer cannot tell by looking.

## Suggestion

Collapse the three client-state parameters into a single explicit context object, e.g.

```ts
interface ICustomizationResolutionContext {
readonly clientChildEnablement?: ...;
readonly clientPlugins?: ...;
readonly mcpServerOwners?: ...;
}
```

Even keeping the members optional, a named object gives one documented place describing what a caller is expected to provide and why, and makes "which views of client state does this call site have?" reviewable at a glance. Better still if the shape can be made required-with-explicit-empty, so forgetting one is a type error.

Low risk, mechanical, and it would have made the Codex gap impossible to introduce.