fix: require account on update_media_buy, flatten preview_creative schema

[bokelley]

Summary

Implements two pre-3.0 schema changes discovered during createAdcpServer implementation (#537):

• update_media_buy: add required account field for governance parity with create_media_buy #2174: Add required account field to update_media_buy — enables automated governance and account resolution on the most financially sensitive mutation tool, matching create_media_buy's shape
• preview_creative: flatten union schema into single object with request_type discriminant #2175: Flatten preview_creative from oneOf union (3 variants) into a single object with request_type discriminant — every tool now works with server.tool(name, schema.shape, handler)

Changes

Schema (static/schemas/source/):

• update-media-buy-request.json: added account (required) referencing account-ref.json
• preview-creative-request.json: replaced oneOf with flat object + allOf conditional validation (if/then for mode-specific required fields)

Server (server/src/):

• adcp-tools.ts: added account validation on update_media_buy, added validate function for preview_creative mode-specific requirements, updated quick-reference hint text
• training-agent/task-handlers.ts: added account to required array, updated preview_creative tool definition (added variant to enum, renamed creatives → requests, added missing fields), added variant mode handler

Docs (docs/):

• update_media_buy.mdx: added account to parameter table, updated all 15 code examples (JS, Python, JSON)
• preview_creative.mdx: replaced 3 per-mode parameter tables with single flat table

Closes #2174, closes #2175.

Test plan

• All 587 unit tests pass (training-agent, storyboards, creative-agent, etc.)
• TypeScript typecheck passes
• Mintlify docs validation passes (no broken links)
• Pre-push hooks pass (version sync, accessibility)
• Verify @adcp/client storyboard media_buy_state_machine.yaml sample_requests are updated to include account (separate client PR)

🤖 Generated with Claude Code

[bokelley]

Reviewed from the @adcp/client side. Both changes look correct and we've already picked them up:

• update_media_buy account: We updated hasAccount: true in createAdcpServer's tool metadata. Account resolution now runs automatically on update_media_buy, matching create_media_buy.

• preview_creative flattening: Once we regenerate schemas from the updated spec, PreviewCreativeRequestSchema becomes a z.object() and we can add previewCreative to CreativeHandlers in createAdcpServer. Currently blocked on schema regen (tracked in our PR).

One minor note: in task-handlers.ts line 2566, request_type is typed as optional (request_type?: 'single' | 'batch' | 'variant') but the schema now has required: ['request_type']. The runtime validation catches it, but the TypeScript type should match. Not blocking.

The validate function for preview_creative (checking mode-specific required fields) is a nice touch — mirrors exactly what the JSON Schema if/then does, but at the application level for better error messages.