fix(decisioning): strip permitted_billing leak from INVALID_BILLING_MODEL details

[bokelley]

Summary

• Strip details.permitted_billing and the enumerated modes from the human message in validate_billing_for_agent. The AdCP billing-not-permitted-for-agent details schema forbids carrying the agent's full permitted-billing subset on the wire — the recognized caller already knows its own capabilities, structured echo is non-conformant.
• Update the validate_billing_rejects_passthrough_only_with_agent_billing test to assert the leak is gone (no permitted_billing key, no ['operator'] rendering, no "permitted modes" phrasing).

Scope

This is the partial fix (Path A) from #375 — the one-line spec-leak fix that's independent of the larger code-rename question. It is not breaking for adopters that aren't reading details.permitted_billing.

The full code rename (AGENT_SUSPENDED / AGENT_BLOCKED / REQUEST_AUTH_UNRECOGNIZED_AGENT / INVALID_BILLING_MODEL → spec-conformant codes) is deferred behind AdCP v3.1, which needs to add AGENT_SUSPENDED / AGENT_BLOCKED / BILLING_NOT_PERMITTED_FOR_AGENT to the error vocabulary with proper recovery semantics before the SDK can switch. Issue #375 is labeled v3.1 to track that follow-up.

Test plan

• validate_billing_rejects_passthrough_only_with_agent_billing updated and verified manually (conftest broken on this workspace from a2a-sdk migration; ran the assertions directly against the function — all pass)
• validate_billing_rejects_advertiser_when_not_in_capabilities still passes (only checks code and message-contains-"advertiser")
• validate_billing_accepts_permitted_modes still passes
• ruff check clean
• mypy src/adcp/ clean

Refs #375

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[bokelley]

Superseded by #393, which landed on main with the comprehensive Path A + Path C fix (full rename to BILLING_NOT_PERMITTED_FOR_AGENT with recovery="correctable", details.rejected_billing + optional details.suggested_billing, no agent_url in details). After merging origin/main into this branch, the content delta is empty.

Closing #375 follow-ups landed in #393.

[bokelley]

Acknowledged — #393 landed the comprehensive Path A + Path C fix, making this PR's delta empty. Nothing further to action here.

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