Remove the unused identity-token path (#161)

## Summary

Removes the unused identity-token path from the merchant SDK, matching
the Node package.
- **`Checkout`:** the gate option for it, its offline verifier and key
cache, the per-issuer policy override that applied only to it, and its
deny bodies. A gate is back to wallet and operator-token identity only,
with its own compliance fields sent as before.
- **Adapters:** the dedicated gates and accessors in all six framework
adapters (FastAPI, Flask, Django, AIOHTTP, Sanic, ASGI).
- **Advertising:** the A2A agent-card extension builder, and the token
path in the agent-memory hint, missing-identity instructions,
`llms.txt`, `skill.md` and OpenAPI security schemes.
- **`has_identity_header`:** no longer counts that header.
- **Dependency:** takes `agentscore-py` >=2.8.0, which drops the same
input.

Version 3.2.0: a minor, because no merchant uses it. The content-type
passthrough on deny responses stays, since a merchant's own `on_denied`
can still set one.

Worked with Varun, who asked for the feature to be removed everywhere.

## Type of change

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

## Public API

Removes the gate option and its exported verifier, signing, key-cache
and type surface, the adapter gates and their accessors, the agent-card
extension builder and URI, and the token flag on the discovery builders
(`agentscore_security_schemes` now takes no arguments).

## Test plan

ruff check, ruff format --check, ty, vulture (as CI runs it) and pytest
(1673 passed, 4 skipped, coverage 97.46%), all green locally. A
replacement test pins that only an operator token or a wallet address
counts as an identity.

## Checklist

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

Author: vvillait88

agentscore_commerce/__init__.py

@@ -25,7 +25,6 @@
     MppxComposeOutcome,
     PricingResult,
     SettleOutcome,
-    build_aip_trusted_issuers,
     format_pydantic_errors,
     get_identity_status,
     pricing_result,
@@ -60,7 +59,6 @@
     A2A_DEFAULT_TRANSPORT,
     A2A_PROTOCOL_VERSION,
     AGENTSCORE_UCP_CAPABILITY,
-    AIP_A2A_EXTENSION_URI,
     FIXABLE_DENIAL_REASONS,
     UCP_A2A_EXTENSION_URI,
     A2AAgentCard,
@@ -93,7 +91,6 @@
     UCPSigningKey,
     UCPVerificationError,
     VerifyWalletSignerResult,
-    aip_a2a_extension,
     build_a2a_agent_card,
     build_agent_memory_hint,
     build_contact_support_next_steps,
@@ -172,7 +169,6 @@
     "A2A_DEFAULT_TRANSPORT",
     "A2A_PROTOCOL_VERSION",
     "AGENTSCORE_UCP_CAPABILITY",
-    "AIP_A2A_EXTENSION_URI",
     "FIXABLE_DENIAL_REASONS",
     "UCP_A2A_EXTENSION_URI",
     "VERIFICATION_SESSION_HEADER",
@@ -243,10 +239,8 @@
     "WorkOutcome",
     "X402BaseRailSpec",
     "__version__",
-    "aip_a2a_extension",
     "build_a2a_agent_card",
     "build_agent_memory_hint",
-    "build_aip_trusted_issuers",
     "build_contact_support_next_steps",
     "build_default_checkout_rails",
     "build_gate_from_policy",