chore(main): release 5.3.0

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🤖 I have created a release beep boop

5.3.0 (2026-05-12)

Features

• adagents: add validate_adagents_structure helper (#708) (ccd1955)
• adagents: ads.txt MANAGERDOMAIN fallback discovery (#704) (#705) (fddea1a)
• buyer-agent-registry: with_caching factory auto-invalidates on mutations (#692) (de65957)
• server: boot validator for webhook_signing.supported capability invariant (#695) (7e9a734)
• testing: add SellerA2AClient for in-process A2A handler testing (#694) (2d1ae2f)
• testing: add SellerTestClient for in-process handler testing (#666) (965eeda)
• types: Sequence[T] on response-only list fields for covariant adoption (#635) (19be8db)

Bug Fixes

• ci: v3 reference seller storyboard job actually asserts on results (#693) (27a5866)
• compat: extract hostname from brand_manifest URLs with paths (#679) (6bb2c26), closes #677
• compat: handle inline BrandManifest object in v2.5 adapters (#685) (edd7d0a)
• compat: warn on non-standard brand_manifest path in inline-object branch (#688) (840e6b3), closes #687
• compat: warn when brand_manifest non-standard path is flattened to domain (#686) (0568d2e)
• server: preserve Starlette lifespan when public_url is callable (#680) (8632847), closes #676

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[github-actions]

🤖 Created releases:

• v5.3.0

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