Add result helpers to Payment and Operation

[loevgaard]

Part of #10 (finding 4).

Why

"Was the capture approved?" meant hand-rolling the same inspection in every integration: find the operation with type=capture, pending=false and qp_status_code='20000'. That's exactly where consumers make bugs — forgetting pending, comparing the code as an int, summing rejected operations, picking "the last operation" from an array whose order they don't control.

What

Operation

• QP_STATUS_APPROVED = '20000'
• isApproved() — !pending && qpStatusCode === '20000' (so a pending op with a stale code, a rejection, or 3-D Secure required are all false)
• isOfType(OperationType|string)

Payment

• operation(int $id): ?Operation
• latestOperation(): ?Operation — highest id, regardless of array order
• operationsOfType(OperationType|string): list<Operation>
• hasPendingOperation(): bool — the "don't read the outcome yet" signal for async operations
• authorizedAmount() / capturedAmount() / refundedAmount() — sums of approved operations only (authorizedAmount() also counts recurring, which is the authorization of a recurring payment)
• isCancelled() — an approved cancel exists

All additive; no existing signature changes. README gains a "Reading what happened to a payment" section (also explaining accepted, balance, and that operation ids are per-payment sequence numbers → a good callback idempotency key).

Tests: unit tests on hand-built payments (approved/pending/rejected mixes, unordered operations, missing amounts, no operations) plus assertions on the mapped payment.json fixture.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.64%. Comparing base (cc4b95c) to head (9a42b4e).

Additional details and impacted files

@@             Coverage Diff              @@
##                1.x      #13      +/-   ##
============================================
+ Coverage     98.50%   98.64%   +0.14%     
- Complexity      147      171      +24     
============================================
  Files            24       24              
  Lines           401      443      +42     
============================================
+ Hits            395      437      +42     
  Misses            6        6              

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