fix(payment): consolidate duplicate refund method and support flexible payloads

[tushar-hub]

Summary

• In razorpay/resources/payment.py, the Payment class previously contained two conflicting definitions of the refund method:
  1. def refund(self, payment_id, amount, data={}, **kwargs)
  2. def refund(self, payment_id, data={}, **kwargs)
• Because Python evaluates class bodies sequentially, the second definition silently overwrote the first.
• This PR consolidates both definitions into a single, robust, backward-compatible refund method:
  • Supports positional amounts (client.payment.refund(payment_id, 2000)).
  • Supports options dictionaries (client.payment.refund(payment_id, {"amount": 2000, "speed": "normal"})).
  • Supports positional amount with additional options (client.payment.refund(payment_id, 2000, {"speed": "normal"})).
  • Supports keyword amount (client.payment.refund(payment_id, amount=2000)).
  • Supports full refund with no extra arguments (client.payment.refund(payment_id)).
  • Replaces dangerous mutable default argument data={} with None and non-mutating payload construction.
  • Fixes copy-paste docstring on fetch_refund_id.
  • Fixes Duplicate refund Function Definitions Overwriting Each Other #289.

Motivation & Impact

• Prevents TypeErrors & Serialization Bugs: Previously, passing a positional integer amount to refund() serialized the raw integer instead of an object with "amount", while passing amount as a keyword argument threw TypeError: refund() got an unexpected keyword argument 'amount'.
• Eliminates Shared Mutable State: Eliminates the classic Python anti-pattern where in-place modification of default data={} persists across calls.
• 100% Backward Compatible: All existing invocation patterns across both v1 and v2 SDK usage continue to work as expected without breaking existing consumers.

Testing Done

• Ran python -m unittest on the full suite: 162 tests passed, 0 failures, 1 skipped.
• Asserted serialized JSON request body in test_refund_create and test_payment_refund.
• Added 4 dedicated test cases to tests/test_client_payment.py:
  • test_payment_refund_with_amount_and_options: verifies refund(payment_id, 2000, {'speed': 'normal', 'receipt': '#rec_1'}) sends {"amount": 2000, "speed": "normal", "receipt": "#rec_1"} and does not mutate caller dictionary.
  • test_payment_refund_with_keyword_amount: verifies refund(payment_id, amount=3000) sends {"amount": 3000}.
  • test_payment_refund_with_keyword_amount_and_data: verifies refund(payment_id, amount=3000, data={'speed': 'optimum'}) sends {"amount": 3000, "speed": "optimum"}.
  • test_payment_refund_without_arguments: verifies refund(payment_id) sends {}.
• Verified zero whitespace/lint issues via git diff --check.