docs(sender): document optional senderFeeAddress on V2 create order

[onahprosper]

Description

Documents the optional senderFeeAddress field on V2 POST /v2/sender/orders, aligned with aggregator PR #972.

What changed for integrators

• When the effective sender fee is greater than zero (senderFee, senderFeePercent, or a profile-configured fee), a fee recipient address is required — either configured on the sender profile or passed as senderFeeAddress on the create payload.
• When the effective fee is zero, senderFeeAddress is not required.
• senderFee and senderFeePercent remain mutually exclusive.

Docs updated

• openapi-v2.yaml — add senderFeeAddress; clarify fee field descriptions (API reference page pulls from this)
• implementation-guides/sender-api-integration.mdx — request fields table, Sender Fees section with onramp examples, v1 vs v2 table
• resources/changelog.mdx — Q3 2026 entry; correct historical v2 sender-fee row
• concepts/participants.mdx — sender fee model note

No breaking API or contract changes; documentation only.

References

• Aggregator: https://github.com/paycrest/aggregator/pull/972
• Jira: KAN-728

Testing

• Review OpenAPI V2PaymentOrderPayload and MDX examples against aggregator resolveV2SenderFee behavior

• Confirm Mintlify preview renders Initiate Order (v2) params and Sender Fees section with examples

• No automated tests (docs-only change)

• This change adds test coverage for new/changed/fixed functionality

Checklist

• I have added documentation and tests for new/changed functionality in this PR
• All active GitHub checks for tests, formatting, and security are passing
• The correct base branch is being used, if not main

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