test(python/svm): add unit tests for ExactSvmSchemeV1 facilitator

[0xAxiom]

Summary

Add 41 unit tests for python/x402/x402/mechanisms/svm/exact/v1/facilitator.py — the only file in mechanisms/svm/exact/v1/ without a dedicated test module after #151 landed test_exact_v1_client.py. test_facilitator.py covers ExactSvmFacilitatorScheme (V2) and only borrows ExactSvmSchemeV1 for one shared-cache cross-version case; the V1 verify/settle pipelines themselves had no direct coverage.

Coverage

• Constructor + attributes — scheme, caip_family, default-vs-injected SettlementCache.
• get_extra / get_signers — multi-address selection, fresh-list return.
• V1 top-level gates — scheme mismatch, network mismatch (V1 stores both at the payload top level, unlike V2).
• feePayer extra gates — missing / None / non-string / unmanaged paths all surface the expected invalid_reason without touching the transaction bytes.
• Transaction decode failures — invalid base64 and empty-base64 payload both surface ERR_TRANSACTION_DECODE_FAILED before instruction parsing.
• Real V1 transaction happy path — the verify pipeline runs against bytes built by the V1 client (ExactSvmClientV1) with mocked RPC.
• V1-specific max_amount_required gate — below threshold returns ERR_AMOUNT_INSUFFICIENT with payer surfaced; exact threshold accepts.
• Mint mismatch / recipient ATA mismatch / fee_payer_transferring — all exercised against the real V1 transaction bytes.
• Memo extra match/mismatch — accepts when memo matches; returns ERR_MEMO_MISMATCH with payer when it doesn't.
• Simulation/sign failure — both surface ERR_SIMULATION_FAILED with the underlying error string in invalid_message.
• Compute-budget price cap — exercised by patching MAX_COMPUTE_UNIT_PRICE_MICROLAMPORTS; sanity guard pins the constant at 5_000_000.
• Settle short-circuit — verify failure surfaces network from payload.network (V1 differs from V2 here).
• Settle happy path — returns mockSignature123 with send_transaction + confirm_transaction ordering captured.
• Settle error paths — send/confirm/missing-feePayer all surface ERR_TRANSACTION_FAILED with the original exception string in error_message.
• Settle feePayer propagation — pulls from requirements.extra into sign_transaction.
• Duplicate-settlement cache — same-instance + cross-instance shared-cache.
• Helper round-trip smoke — V1 client output decodes to the 3–6 instruction range the facilitator accepts.

Test plan

• uv run pytest tests/unit/mechanisms/svm/test_exact_v1_facilitator.py — 41/41 passing
• uv run pytest tests/unit/mechanisms/svm/ — 161/161 passing (no regressions in sibling suites)
• uv run pytest tests/unit/ — 852/852 passing
• uv run ruff format --check . — 191 files clean
• uv run ruff check . — clean
• Changeset fragment added at python/x402/changelog.d/python-svm-exact-v1-facilitator-tests.doc.md
• Commit GPG-signed

[cb-heimdall]

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[0xAxiom]

Friendly ping — this PR has been open for 16 days awaiting review. Happy to address any feedback or rebase if needed. Thanks for your time!