Reliable Reporting: report-definition contract_version 1.2 requires a package_id dimension for media buys #8971

Problem

A Reliable Reporting revision for a media buy can carry no package dimension and still be fully valid. reporting-report-definition.json declares dimensions as a free list of strings, and the reference fixture static/compliance/source/test-vectors/reporting-reconciliation/report-definition.json itself uses ["date", "media_buy_id"].

Every media buy has at least one package (create_media_buy takes packages with minItems: 1, or a proposal_id that creates them), and packages in one buy can carry different rates, pricing models and budgets. A buyer that bills, caps or reconciles per package cannot split a buy-level revision back into packages without guessing.

Proposal: report-definition contract_version 1.2

Add contract_version: "1.2" to reporting-report-definition.json. A 1.2 definition whose revisions cover media buys MUST declare a package_id dimension, so every row carries the buyer-visible package.

This follows the pattern 1.1 already uses (it requires official_correction_mode only when contract_version is "1.1"):

{
  "if": { "properties": { "contract_version": { "const": "1.2" } }, "required": ["contract_version"] },
  "then": { "properties": { "dimensions": { "contains": { "const": "package_id" } } } }
}

1.2 keeps 1.1's requirements.

Compatibility

Non-breaking, so it can ship as a minor in 3.x: 1.0 and 1.1 definitions stay valid and the existing reference fixture is unchanged. Only sellers that adopt 1.2 take on the requirement. A new 1.2 fixture with package-grain rows (media_buy_id, package_id, date) would cover it. Buyers that need package rows can require 1.2 when they enroll a seller's Reliable Reporting offering.

Making package_id required for every media-buy definition, whatever its version, would be a 4.0 change (see #8973 for the matching by_package rule).

Open questions

1. Is there media-buy reporting that is legitimately not package-scoped (an account-level summary, for example)? If so, should the 1.2 rule key off media_buy_ids on the revision, or a declared grain?
2. Dimension name: package_id (matching media_buy_id in the reference fixture), or package (as subscription selectors use today)?

Related: #8970 (3.x by_package guidance), #8973 (4.0 by_package requirement).