chore(decisioning): AuthInfo migration to kind-discriminated credential variant

[bokelley]

Summary

Sub-task of Tier 2 but deserves its own tracking. Current AuthInfo is flat (kind: str + key_id + principal + scopes). Migrate to a credential: ApiKeyCredential | OAuthCredential | HttpSigCredential discriminated union per JS ResolvedAuthInfo.

Design

• New shape: AuthInfo.credential is a discriminated union keyed by kind
• Keep flat shape as deprecated alias for one minor with DeprecationWarning
• Existing adopter context_factory hooks continue to work via wrap-around shim
• Removal in next minor after deprecation lands

Compatibility

Wrap-around shim translates flat → discriminated for legacy context_factory callsites. Adopters migrate at their own pace within the deprecation window.

Cross-references

• Parent: Tier 2 BuyerAgentRegistry (filed separately as feat(decisioning): BuyerAgentRegistry Protocol + AuthInfo discrimination + billing_capability enforcement (Tier 2) #349)
• RFC: docs/proposals/v3-identity-bundle-design.md
• JS counterpart: ResolvedAuthInfo in adcp-client#1269

🤖 Generated with Claude Code

[bokelley]

Triage

Classification: Feature request (breaking — multi-minor deprecation cycle)
Bucket(s): client (label not present in repo — gap noted)
Status: deferred

Blocked-on: #349 — resurfaces on merge

The ApiKeyCredential | OAuthCredential | HttpSigCredential discriminated union this issue migrates AuthInfo into is itself defined by the open parent #349 ("BuyerAgentRegistry Protocol + AuthInfo discrimination"). The wrap-around shim and deprecation alias described here cannot land before that union type exists on main. Structurally, this issue is the "tracked separately" companion that #349's body explicitly flags.

My take: Even once #349 lands, the removal-in-next-minor step is a public breaking change (flat AuthInfo fields disappear from a @dataclass that adopters access directly), so the full migration cycle will need an explicit sign-off before the execute gate opens.

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Triaged by Claude Code. Session: https://claude.ai/code/session_011PGotVENidpbb36UBK3LoE

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Generated by Claude Code