Update API models and prepare 3.1.2

[TeaDrinkerPlus]

Adds the optional PaymentFactor.storePaymentMethodForBuyer field for supported pay scenarios, PaymentResultInfo.fingerprint, authorization consult periodic-signing models, and createMandate request/response support. Updates the SDK version and README to 3.1.2.

The storage flag is documented for enabled pay scenarios; its presence in the shared model does not establish consult/createPaymentSession support.

Validation

• Tested commit: ea1ad79d2495d12f27dfef94a7156ddfea6ab46b.
• Fetched and merged latest origin/master before submission; already up to date. Commit/tree remain identical to the tested source. Version files and README rechecked after synchronization.
• Offline public/business regression: 32/32 passed, including the new storage-flag serialization case.
• Sandbox: 4/5 passed; Meter uploadEvent returns F/PARAM_ILLEGAL. Pay, createPaymentSession, payment inquiry, and product image upload/cleanup passed.
• Java JSON (13), transport (3), and four Jackson compatibility rounds passed. Adding model fields changes all-argument constructor signatures; no-argument/property-based usage remains supported.

Unresolved sandbox result

Java, Python and PHP Meter tests fail with PARAM_ILLEGAL; diagnostics show CUSTOMER_NOT_FOUND although customer inquiry returns ACTIVE. Waiting 15/60 seconds did not resolve it. The exact Python SDK commit that passed on September 18 also fails when rerun today with the identical test script. The cause remains unresolved; this is not a fully passing release acceptance. Go/.NET Meter were not executed under the existing scope.

Buyer-card persistence and live periodic-signing/createMandate behavior were not exercised. No publication or merge is included in this PR submission; owner review and manual merge are required.

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