Let CreateSessionOnMissing mint a sign_in session

[vvillait88]

Summary

CreateSessionOnMissing gains an optional kind ("kyc" | "sign_in") that rides through to POST /v1/sessions. A gate running an empty compliance policy that only needs an account to key durable state on (a prepaid balance, say) could previously auto-mint only the KYC kind, which asks the buyer for identity documents nothing in that merchant ever checks; sign_in is registration-only and still yields an operator token. When the kind is sign_in the minted denial carries a message describing an account sign-in instead of the KYC default, so a merchant using the default 403 body does not tell the agent something false. When kind is None nothing changes: the body omits it and the API default applies. Parity with the node library change of the same day. Takes agentscore-py 2.6.9, which carries the option.

Type of change

• Bug fix (no breaking change)
• New feature (no breaking change)
• Breaking change (existing callers must update)
• Docs, tests, or internal maintenance only

Public API

CreateSessionOnMissing.kind: Literal["kyc", "sign_in"] | None = None (new, optional). No migration for existing callers.

Test plan

tests/test_sessions.py: a new case asserts the POST body carries kind: "sign_in" and that the returned reason's message talks about signing in with an AgentScore account and not about KYC; the existing forwarding case additionally asserts kind is absent when unset. Locally: uv run ruff check, uv run ruff format --check, uv run ty check agentscore_commerce/, uv run pytest (1869 passed, 4 skipped, 95.37% coverage against the 95% floor).

Checklist

• Tests cover the new behavior, and the suite passes locally
• Lint, format, and type checks pass
• Docs and README examples updated if the public surface changed
• No secrets, credentials, or personal data in the diff or the tests

Worked with Varun. Version bumped to 2.9.0; the tag follows the merge. The README carries no CreateSessionOnMissing example today, so the field's docstring is its documentation.

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