feat(spec): deprecate top-level max_results on get_signals, pin pagination precedence

[bokelley]

Closes #3113

Summary

get-signals-request.json was the only paginated read endpoint carrying both a legacy top-level max_results (no cap, no default) and the standard pagination envelope (pagination.max_results, max: 100, default: 50). The schema was silent on which wins when both are present — PR #3109 implemented pagination wins in code without a normative spec anchor.

This PR adds the anchor:

• max_results: Marked "deprecated": true + description-prefix deprecation notice per the repo's established pattern (mirrors targeting.json). Includes MUST-level precedence rule and guidance for sellers receiving only the legacy field. Queues removal for AdCP 4.0.
• pagination: Added a sibling description (annotation-only in draft-07 strict semantics; this repo's existing account field uses the same pattern) restating the precedence rule at the envelope level.
• docs/signals/tasks/get_signals.mdx: Updated parameter table and three code examples to use pagination.max_results instead of the top-level legacy field.

Schema audit: get_products, list_creatives, list_creative_formats, get-collection-list, get-property-list, get-media-buy-artifacts, and tasks-list all carry only pagination — no other endpoint has this drift.

Non-breaking justification: Description changes and a deprecated: true annotation. No type, structure, or required-field changes. No existing wire shapes are rejected. The MUST-level precedence rule documents what PR #3109 already implements; it creates a new conformance obligation but does not break any caller that was already following the pagination-wins behavior.

Callers to note: Any implementation sending max_results > 100 at the top level will, once it adopts the new precedence rule, shift to pagination.max_results's maximum of 100. This is a behavioral cap change for those callers and is called out in the changeset.

Pre-PR review

• code-reviewer: approved — no blockers. Two nits noted: (1) description sibling to $ref is annotation-only under draft-07 strict semantics — acknowledged, matches existing account field pattern in this file; (2) pagination description inline-repeats max: 100, default: 50 from pagination-request.json — acceptable for readability, minor divergence risk if pagination-request.json changes.
• ad-tech-protocol-expert: approved — non-breaking per spec. "deprecated": true boolean added to match repo pattern. Precedence rule covers both conflict case and solo-legacy-field case. Changeset bump level minor is correct (new MUST-level conformance obligation).

Follow-on (not in this PR)

The storyboard get_signals_pagination_integrity (being added in PR #3109) should gain a conflict-test step that sends both max_results and pagination.max_results with differing values and asserts the response honors pagination.max_results. Blocked on #3109 merging. Filed as a nit from the product-expert review.

Session: https://claude.ai/code/session_01Um5nGVYLUUJvNp1EFN9W89

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Generated by Claude Code

[bokelley]

Independent review — ready to merge.

Verified against expert recommendations

• ✅ Non-breaking shape. Adds `deprecated: true` annotation + description prose; doesn't remove the field. Matches the existing precedent at `static/schemas/source/core/product-filters.json:131-132` so `npm run test:schemas` tolerates the annotation under draft-07.
• ✅ Changeset type. `minor` is correct — adds a schema-visible spec change (the `deprecated` annotation + description-level MUST). Not `patch` (which would be wrong for a spec annotation), not `empty` (which is reserved for non-protocol changes).
• ✅ Precedence pinned in two places. Schema description AND task-reference docs both say "pagination.max_results takes precedence." SDK code-gen consumes the schema description; humans read the docs.
• ✅ Cross-endpoint audit. Confirmed in the changeset: `get_products`, `list_creatives`, `list_creative_formats`, etc. carry only `pagination`. No other endpoint has the duplicate-field shape — `get_signals` was the lone outlier.
• ✅ 3.x-friendly. Doesn't break 3.x callers; flagged for removal in 4.0 per the deprecation prose.

Compatibility with #3109

The training agent's `handleGetSignals` (#3109) already implements the precedence this spec change pins (pagination wins). My follow-up commit preserved the legacy 50-cap on top-level `max_results` while honoring the schema's 100-cap on `pagination.max_results`; the 50-cap is conservative under the new SHOULD-level "subject to a maximum of 100" — agents may cap below the ceiling. No code change needed in the training agent.

CI is green. Ready.