Fix x402 URL route balance selection

[Abiorh001]

Summary

• keep direct x402 URL payments out of Gateway/Circle balance preflight and delegate them to the x402 adapter
• keep nanopayment/Gateway URL routes on Gateway balance checks
• normalize preferred URL route matching for enum and string inputs in the router

Verification

• uv run ruff check src/ && uv run ruff format --check src/
• uv run ruff check src/omniclaw/client.py src/omniclaw/payment/router.py tests/test_client.py tests/test_payment_router.py
• uv run ruff format --check src/omniclaw/client.py src/omniclaw/payment/router.py tests/test_client.py tests/test_payment_router.py
• uv run pytest tests/test_client.py::TestUrlRouteBalanceChecks tests/test_payment_router.py tests/test_x402_sdk_adapter.py -q
• uv run pytest -q (652 passed)
• Live external seller + private hosted facilitator: GatewayWalletBatched verify/settle 200 on eip155:5042002
• Live external seller + private hosted facilitator: exact_evm verify/settle 200 on eip155:5042002

Review

• Reviewer agents rechecked the direct x402 preflight fix and route normalization; no remaining blockers reported.

Note: examples/agent/buyer/policy.json has an unrelated local worktree change and is intentionally not part of this PR.