docs(reference/migration): fix url_type tracking_pixel → role: impression_tracker in creatives.mdx

[bokelley]

Closes #3692

The asset discovery example at docs/reference/migration/creatives.mdx:213 used "url_type": "tracking_pixel" inside a requirements block — wrong on two counts: url_type is not a property of url-asset-requirements.json (it lives on the manifest-side asset payload in url-asset.json), and tracking_pixel is not a valid url-asset-type.json enum value (valid: clickthrough / tracker_pixel / tracker_script). The correct constraint for this URL slot in a format definition is "role": "impression_tracker", using the role enum from url-asset-requirements.json. This goes in the same direction as #3670 / #3671 / #3673 — aligning migration docs with actual schema shapes so sellers following the docs produce conformant output.

Non-breaking justification: docs-only change; no schema or wire format was modified. url-asset-requirements.json already has additionalProperties: true, so the old example validated silently but with no enforced semantic. The fix corrects the teaching example to use the schema-correct field.

Note on dist/docs/ occurrences: grep finds five matching strings in dist/docs/3.0.{0,1,2}/ and dist/docs/3.0.0-rc.{1,2}/. These are immutable versioned snapshots generated by release-docs.yml on each publish and are intentionally left unchanged — they are the historical record of what shipped, not source files.

Pre-PR review:

• docs-expert: approved — "role": "impression_tracker" is the correct field and value per url-asset-requirements.json; blocker from first pass (wrong field) resolved in second pass; no remaining blockers
• code-reviewer: approved — tracker_pixel enum value confirmed correct; --empty changeset correct for docs-only; dist/docs/ occurrences are release snapshots, intentionally out of scope

Triage-managed PR. This bot does not currently iterate on
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See #3121
for context.

Session: https://claude.ai/code/session_01XUwgQ1QoFMErF3k5A65AeJ

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