spec(conformance): rejection-arm vs errors[] mutual-exclusion test + storyboard alignment (closes #3998, #3914)

[bokelley]

Summary

The wire-placement guidance on `GOVERNANCE_DENIED` (shipped to `main` via #3929 and to 3.0.x via #3996) is normative MUST-language: when a task response defines a structured rejection arm (`AcquireRightsRejected`, `CreativeRejected`), the arm IS the canonical denial shape — sellers MUST NOT additionally emit the error code in `errors[]` or `adcp_error`. The schema enforces this with `not: { required: ["errors"] }` on each rejection arm.

Until now the rule was asserted only in prose. This PR adds executable conformance and aligns the existing storyboards with the rule.

Changes

`tests/rejection-arm-mutual-exclusion.test.cjs` — schema-validation conformance check (new)

Asserts both directions of the rule for both rejection arms:

• Accept: canonical rejection-arm shape (status + reason, no errors[])
• Reject: rejection-arm with errors[] populated — the schema's `not: { required: ["errors"] }` clause must fire

Catches regression on the constraint before any storyboard does. Wired into the aggregate `npm test` run via `test:rejection-arm-mutex`.

`brand_rights/governance_denied` storyboard — assertions corrected

Was asserting:
```yaml

• check: error_code
  value: "GOVERNANCE_DENIED"
  ```

…on a task whose canonical denial shape is the `AcquireRightsRejected` arm (`status: "rejected"` + `reason`). Adopters returning the spec-correct shape were failing the storyboard. Now asserts:

```yaml

• check: response_schema # AcquireRightsRejected validates; not: required: errors fires on dual-emission
• check: field_value
  path: "status"
  value: "rejected" # discriminator on the rejection arm
• check: field_present
  path: "reason" # governance findings propagated verbatim
  ```

Also drops the `expect_error: true` and `negative_path: payload_well_formed` flags — Case-1 of the rule explicitly says transport-level success markers MUST NOT be flipped (HTTP 200 / MCP `isError: false` / A2A `succeeded`). The narrative is updated to spell out that the schema's `not: { required: ["errors"] }` rule is what enforces the no-dual-emission discipline.

Closes the storyboard portion of #3914 (the storyboard was non-spec; adopters returning the rejection arm were failing it).

`media_buy_seller/governance_denied` storyboard — narrative tightened

The assertions were already correct (Case-2 of the rule: `create_media_buy` has no rejection arm, so `errors[].code: GOVERNANCE_DENIED` + `adcp_error.code: GOVERNANCE_DENIED` + flipped transport markers is the canonical denial shape). Updated the `expected` prose to make Case-2 explicit and cross-reference the brand-rights scenario as the Case-1 counterpart, so a reader sees both arms of the rule from either entry point.

Wire-format impact

None. Schema constraints unchanged. Pure conformance + documentation: the schema rule was already in place; this PR makes it discoverable from a failing test and aligns the existing storyboards with the rule.

Test plan

• `npm run test:rejection-arm-mutex` — 4/4 passing locally (canonical accept + dual-emission reject for both rejection arms)
• Compliance bundle rebuilds cleanly (`node scripts/build-compliance.cjs`)
• CI green
• Conformance test runs as part of the aggregate `npm test` suite

Refs

• Add a storyboard exercising rejection-arm vs errors[] mutual-exclusion for GOVERNANCE_DENIED / CREATIVE_REJECTED #3998 — issue this PR closes (rejection-arm mutex conformance)
• brand_rights/governance_denied storyboard: expect_error: code: GOVERNANCE_DENIED is non-spec — AcquireRightsRejected is the canonical denial shape #3914 — storyboard portion closed by this PR (was filed for the brand_rights/governance_denied assertion drift)
• spec(errors): wire-placement guidance for GOVERNANCE_DENIED and GOVERNANCE_UNAVAILABLE #3929 — `main`-side wire-placement guidance this PR enforces
• 3.0.6 cherry-picks: governance wire-placement + ctx_metadata reservation + storyboard fixture fixes #3996 — 3.0.6 cherry-pick of the same guidance

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