Fix: Spell receiptUrl the way the 2.1 schemas do

[OSkrk]

Current behavior

snake_to_camel_case rewrites any key ending in _url to ...URL. That is correct for
responderURL, and ocpp_csms_url is already special-cased just above it, but OCPP 2.1
spells the NotifySettlement field receiptUrl:

>>> from ocpp.charge_point import snake_to_camel_case
>>> snake_to_camel_case({"receipt_url": "https://example.com/receipt/1"})
{'receiptURL': 'https://example.com/receipt/1'}

The library therefore produces a payload its own validation rejects, in both directions:

jsonschema.exceptions.ValidationError:
Additional properties are not allowed ('receiptURL' was unexpected)

Failed validating 'additionalProperties' in schema:
    {'$id': 'urn:OCPP:Cp:2:2025:1:NotifySettlementResponse',
     'properties': {'receiptId': {...}, 'receiptUrl': {...}}, ...}

In practice a charging station reporting a settled payment can neither send the receipt
URL it generated nor relay the one the CSMS returned — NotifySettlementRequest and
NotifySettlementResponse both carry receiptUrl, so both fail with FormatViolation
and the receipt is lost. Found on a live 2.1 link between a charging station and a CSMS,
where the CSMS answered with a correct receiptUrl and the station's own schema check
refused it on the way back.

The comment above these lines already notes that the spec is inconsistent about URL
casing; receipt_url is simply a case that was never added.

New behavior

One replacement alongside the existing ocpp_csms_url case:

key = key.replace("ocpp_csms_url", "ocppCsmsUrl")
key = key.replace("receipt_url", "receiptUrl")
key = key.replace("csms", "CSMS")
key = key.replace("_url", "URL")

camel_to_snake_case already reverses receiptUrl correctly, so nothing was needed
there; a test covers both directions.

Impact

No breaking change, and nothing to do for users.

• receiptUrl, responderURL and ocppCsmsUrl are the complete set of URL-shaped keys
  across the 1.6, 2.0.1 and 2.1 schemas. responderURL and ocppCsmsUrl are untouched
  and serialise exactly as before; this completes the set.
• 1.6 and 2.0.1 have no receipt field at all, so no message in any earlier version
  goes near this rule. receipt_url appears in exactly two places in the library,
  v21/call.py and v21/call_result.py, both NotifySettlement.
• No schema in any version expects receiptURL, so the old spelling was never valid
  and nothing can have depended on it — anything that produced it was already being
  rejected by payload validation.

Checklist

1. Does your submission pass the existing tests?
   pytest tests passes (tests/test_messages.py needs hypothesis, which my
   environment lacks; it is untouched by this change).
2. Are there new tests that cover these additions/changes?
   One row added to each of the existing test_snake_to_camel_case and
   test_camel_to_snake_case tables, so both directions are covered.
3. Have you linted your code locally before submission?
   black --check, isort --check-only and flake8 --config .flake8 are clean on the
   changed files.

[Copilot]

Copilot review overview

🟢 Approval recommended

The focused change matches the NotifySettlement schemas and has tests for both conversions.

Review effort: Balanced
Findings: None

What changed in this PR

This PR makes URL field conversion match the OCPP 2.1 NotifySettlement schemas, so receipt URLs use receiptUrl.

Changes:

• Special-cases receipt_url during serialization.
• Adds tests for conversion in both directions.

File	Description
tests/​test_charge_point.py	Tests both receiptUrl conversions.
ocpp/​charge_point.py	Serializes receipt_url as receiptUrl.

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