feat(BTI-23): add eMandate solution (B2C + B2B)

[vildanbina]

What

Adds eMandate as a DataRequest-based solution (reached via app.solutions), matching the shape of the PHP and Node SDKs. Covers both service codes — emandate (B2C) and emandateb2b (B2B) — and all five actions.

How

• EmandateBuilder extends SolutionBuilder; EmandateB2BBuilder subclass overrides only the service name (emandateb2b). Both registered in SolutionMethodFactory.
• Per-action required/allowed service-parameter validation via get_allowed_service_parameters, enforced by the existing ServiceParameterValidator before the request hits the wire.
• Reuses PaymentResponse — no bespoke per-action response model (same convention as iDIN / PHP / Node).
• examples/emandate.py (env-gated, B2C + B2B, all five actions) and a README ### eMandate section.

Tests

• Unit + feature tests for every action (success + missing-required-param validation), plus B2B service-name and factory-resolution tests.
• Full suite: 2359 passed, ruff clean.

Live verification (Buckaroo Plaza, test mode)

• GetIssuerList → 190 success (B2C + B2B).
• CreateMandate → 791 pending + redirect URL (B2C + B2B) — correct; the consumer approves at their bank.
• GetStatus / ModifyMandate / CancelMandate were exercised against a placeholder mandate id and returned expected gateway rejections (the real mandate id only exists after the bank redirect completes). Happy paths for those three are not yet verified end-to-end.

Notes

• Decisions: currency kept optional (eMandate has no amount); reference-SDK param names used (debtorReference, mandateId, …).
• Out of scope: SEPA PayWithEmandate (collecting against a mandate), async push handling.

Jira: BTI-23 (epic BTI-552).