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

[0xAxiom]

Summary

Add 28 unit tests for python/x402/x402/mechanisms/svm/exact/v1/client.py — the only file in mechanisms/svm/exact/v1/ without a dedicated test module. test_facilitator.py imports v1.facilitator briefly for shared-cache testing, but the V1 client surface had zero direct coverage.

Coverage

• Constructor: scheme attribute, signer reference, default rpc_url=None, custom rpc_url stored, empty client cache.
• _get_client: CAIP-2 devnet/mainnet → default RPC URL, V1 legacy solana-devnet/solana name normalization to CAIP-2 RPC URL, custom rpc_url priority, per-network client caching across legacy + CAIP-2 inputs (single SolanaClient construction), unsupported CAIP-2 + unsupported legacy network errors surfaced through normalize_network.
• create_payment_payload: missing feePayer in extra, extra=None case, mint not found, unknown token program owner, oversized memo, V1 reads max_amount_required (different amounts → different serialized bytes), Token-2022 mint owner accepted, default random memo divergence across calls, custom memo round-trips into the transaction bytes, non-string memo (int) falls back to random nonce, boundary MAX_MEMO_BYTES accepted.

Test plan

• uv run pytest tests/unit/mechanisms/svm/test_exact_v1_client.py -q → 28 passed
• uv run ruff format clean
• uv run ruff check clean
• Changelog fragment added at python/x402/changelog.d/python-svm-exact-v1-client-tests.doc.md
• Commit GPG-signed

[cb-heimdall]

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