Honor the delivery_mode argument in Exchange.Message

[uttam12331]

Exchange.Message accepts a delivery_mode argument and documents it as:

delivery_mode (bool): Set custom delivery mode. Defaults to :attr:delivery_mode.

but the method body never uses it — it always calls maybe_delivery_mode(self.delivery_mode), so the caller's value is silently discarded:

>>> ex = Exchange('ex', 'direct', delivery_mode='persistent')
>>> ex.Message(body, delivery_mode='transient')['properties']['delivery_mode']
2      # expected 1
>>> Exchange('e2', 'direct', delivery_mode='transient').Message(body, delivery_mode='persistent')['properties']['delivery_mode']
1      # expected 2

So a caller asking for a transient message on a persistent exchange still gets delivery_mode=2 (RabbitMQ fsyncs and persists it), and one asking to persist can silently end up transient.

The sibling Producer._delivery_details already handles this correctly:

return exchange.name, maybe_delivery_mode(
    delivery_mode or exchange.delivery_mode,
)

This change makes Exchange.Message use the same idiom, falling back to the exchange default only when no delivery mode is passed. Exchange.publish() calls self.Message(message) with no delivery_mode, so None or self.delivery_mode reproduces the current behaviour exactly — nothing changes for existing callers.

Added t/unit/test_entity.py::test_Exchange::test_create_message_delivery_mode, which fails before this change and passes after (the argument overrides the exchange default; without it the default is used).

pytest t/unit/test_entity.py t/unit/test_messaging.py → 128 passed; flake8 (max-line-length 117) clean.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.81%. Comparing base (4281680) to head (bf5f25d).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2570   +/-   ##
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  Coverage   82.81%   82.81%           
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  Files          79       79           
  Lines       10333    10333           
  Branches     1187     1187           
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  Hits         8557     8557           
  Misses       1575     1575           
  Partials      201      201           

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