fix(storyboard): proposal-mode enricher respects fixture's explicit packages

[bokelley]

Summary

Follow-up to PR #1603, which introduced proposal-mode in the create_media_buy request enricher by reading context.proposal_id. `context.ts::get_products()` auto-captures `proposals[0].proposal_id` from any `get_products` response that returned `proposals` — essentially every brief flow against a seller that supports proposal-mode discovery.

The enricher fell back to `context.proposal_id` whenever the fixture didn't explicitly set `proposal_id`. That meant a storyboard authoring `packages` directly in `sample_request` would have its packages dropped in favor of the auto-captured proposal — forcing every sales storyboard whose brief returned proposals through the seller's strict proposal-lifecycle validation.

Concrete impact

Surfaced when consuming sellers like `test-agent.adcontextprotocol.org` that enforce proposal-status / IO-acceptance / total_budget rules on `proposal_id`-shaped requests. With this branch's SDK packed locally and consumed by the test-agent's storyboard matrix:

Tenant	Before fix (SDK 6.18.0)	After fix
/signals	✓ 67 / 58	✓ 67 / 58
/sales	✗ 50 / 198 (17 storyboards regressed)	✓ 65 / 254 (2 remain — pre-existing storyboard authoring issues, see below)
/governance	✓ 65 / 102	✓ 65 / 102
/creative	✓ 66 / 118	✓ 66 / 118
/creative-builder	✓ 60 / 100	✓ 60 / 100
/brand	✓ 66 / 45	✓ 66 / 45

Storyboards restored: `sales_guaranteed`, `sales_non_guaranteed`, `schema_validation` (×3), `media_buy_seller` (×11 sub-scenarios), `creative_generative/seller`, and others.

Fix

The enricher reads `context.proposal_id` only when the fixture authors NEITHER `proposal_id` NOR `packages`. Fixture is the storyboard author's intent; respect it.

Fixture	Mode chosen
`proposal_id` set	proposal-mode
`packages` set, no `proposal_id`	package-mode (was: incorrectly proposal-mode via context fallback)
neither	proposal-mode if `context.proposal_id` set; else package-mode

Test plan

• `node --test test/lib/request-builder.test.js` (85/85, 4 new cases)
• `node --test test/lib/request-builder-jsonschema-roundtrip.test.js test/lib/request-builder-schema-roundtrip.test.js test/lib/storyboard-completeness.test.js` (1731/1731 — no regression on roundtrip / completeness)
• `node --test test/examples/hello-seller-adapter-proposal-mode.test.js` (3/3 — original PR fix(storyboard): proposal-mode create_media_buy request shape (closes #1600) #1603 integration coverage still passes)
• `npm pack` + install into adcontextprotocol/adcp test-agent + run `scripts/run-storyboards-matrix.sh` — 5 of 6 tenants meet floors; /sales restored from 50 → 65 clean storyboards. Two remaining failures (`sales_proposal_mode`, `media_buy_seller/proposal_finalize`) are spec-side fixture issues PR fix(storyboard): proposal-mode create_media_buy request shape (closes #1600) #1603 was masking — separate PR will address.

Out-of-scope follow-ups

The two remaining /sales failures are pre-existing storyboard authoring issues that PR #1603's incorrect over-application masked. They live in `adcontextprotocol/adcp` and need fixes there:

1. `sales_proposal_mode` authors `proposal_id: "balanced_reach_q2"` as a literal in two places. The training-agent's seed proposals don't include that id (it seeds `pinnacle_cross_channel`, `viewpoint_multi_screen`, `sparq_social_amplification`, `novamind_ai_audience`). Switch to `$context.proposal_id` to dynamically follow the brief response, matching the pattern `media_buy_seller/proposal_finalize` already uses.
2. `media_buy_seller/proposal_finalize` doesn't include `io_acceptance` in the `create_media_buy` fixture, but the test-agent's seed proposals (post-3.0.7) carry `requires_signature: true` insertion orders. Either the fixture needs `io_acceptance`, or the seed needs to differentiate IO-required vs non-IO proposals.

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