update_media_buy: add required account field for governance parity with create_media_buy

[bokelley]

Problem

create_media_buy has a required account field, but update_media_buy does not. This creates a structural gap:

1. Governance: Automated governance checks need the account to resolve the governance agent. Without it, update_media_buy — the most financially sensitive mutation tool (budget changes, package additions, cancellations) — cannot be governed by the framework.

2. Account resolution: Server frameworks like createAdcpServer resolve accounts automatically for tools with an account field. update_media_buy forces sellers to manually look up which account owns a media buy.

3. Asymmetry: The buyer already knows the account when calling update_media_buy (they created the media buy with one). There's no reason not to provide it.

Proposal

Add account: AccountReference as a required field on UpdateMediaBuyRequest, matching create_media_buy.

This is non-breaking for sellers (they receive more data). Buyers already have the account context from create_media_buy.

Impact

• Enables automated governance on media buy mutations
• Enables automated account resolution in server frameworks
• Parity with create_media_buy request shape

Priority

Should land before 3.0 — after release, adding a required field is a breaking change for buyers.

Discovered during createAdcpServer implementation (#537).