fix(storyboard-runner): create_media_buy enricher drops proposal_id, total_budget, idempotency_key from sample_request

[bokelley]

Root cause

src/lib/testing/storyboard/request-builder.ts:207 — the create_media_buy enricher rebuilds the request from scratch and only spreads start_time / end_time / packages from step.sample_request. It substitutes account with resolveAccount(options) = {brand: {domain: 'test.example'}, sandbox: true} and silently drops:

• proposal_id (sourced via $context.proposal_id after adcp#4088)
• total_budget
• idempotency_key

The YAML's proposal_id: \"\$context.proposal_id\" substitution never reaches the wire because the enricher discards it before send.

Symptom

Proposal-mode adapter gate test masks create_media_buy in EXPECTED_FAILURES (PR #1599). Without the mask, the adapter receives a request without proposal_id, can't look it up, returns an error or wrong-shape response, and fails the schema check.

Fix

The enricher should preserve all sample_request fields and only override what it explicitly needs to (account substitution). Spread step.sample_request first, then layer the runner's substitutions on top.

Acceptance

• All sample_request fields reach the wire when the runner enriches create_media_buy (and any other tool with similar enrichment logic — audit request-builder.ts for the pattern).
• Proposal-mode adapter gate test passes with empty EXPECTED_FAILURES.
• Drop the create_media_buy allowlist entry from test/examples/hello-seller-adapter-proposal-mode.test.js (fix(test): drop stale get_products_refine allowlist on proposal-mode adapter #1599).
• Add a regression test that pins the field-pass-through for at least one mutating tool with discriminator-bearing fields in sample_request.

Refs

• fix(adapter): hello_seller_adapter_proposal_mode create_media_buy fails schema validation against AdCP 3.0.7 #1600 (original misdiagnosis — closed)
• fix(test): drop stale get_products_refine allowlist on proposal-mode adapter #1599 (interim allowlist mask)
• feat(examples): add hello_seller_adapter_proposal_mode for sole-stateful exemption coverage (#1549) #1594 (proposal-mode adapter)
• chore(deps): bump AdCP 3.0.6 → 3.0.7 (closes #1586) #1595 (3.0.7 bump that exposed this via cascade-unmask)
• adcp#4088 (the upstream chaining fix)

[bokelley]

Already fixed by PR #1607 (merged 2026-05-08, commit 6e0bcc3).

Verified in current main:

• src/lib/testing/storyboard/request-builder.ts — create_media_buy (non-proposal branch, line 363) spreads fixtureBody first then layers enricher overrides. The fix was applied to all 5 fixture-aware enrichers (create_media_buy, update_media_buy, get_media_buys, get_media_buy_delivery, comply_test_controller), with envelope fields (context, ext, push_notification_config, idempotency_key) dropped from local spreads so the outer enrichRequest re-injects them with runnerVars for mustache token expansion.
• test/examples/hello-seller-adapter-proposal-mode.test.js:35 — EXPECTED_FAILURES = [] (the create_media_buy allowlist entry was removed).
• test/lib/request-builder.test.js:223 — regression test preserves top-level sample_request fields the enricher does not normalise (#1604) pins buyer_ref, total_budget, currency, and reporting_webhook pass-through.

Issue stayed open because PR #1607 didn't carry a Closes #1604 link in its description. Closing now.