fix(event_source): mypy strict mode compliance

[stenczelt]

Issue number: closes #8331

Summary

Changes

aws_lambda_powertools.utilities.data_classes.event_source complies with mypy in --strict mode as well.

Removed a redundant casting to the dataclass in examples/event_sources/src/kinesis_firehose_response_exception.py.

Changes are localised to event_source.py, rather than trying to annotate lambda_handler_decorator for all use cases, unlike earlier attempts, e.g. #4165. Since the decorator only takes the dataclass as a keyword argument, I've defined a protocol and cast the result to this.

User experience

Having a handler using the @event_source decorator, in main.py

from aws_lambda_powertools.utilities.data_classes import SQSEvent, event_source
from aws_lambda_powertools.utilities.typing import LambdaContext


@event_source(data_class=SQSEvent)
def lambda_handler(event: SQSEvent, _context: LambdaContext) -> None:
    return None

Before:

$ mypy --strict main.py

main.py:5:2: error: Untyped decorator makes function "lambda_handler" untyped  [untyped-decorator]

After:

$ mypy --strict main.py 

Success: no issues found in 1 source file

---

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[boring-cyborg]

Thanks a lot for your first contribution! Please check out our contributing guidelines and don't hesitate to ask whatever you need.
In the meantime, check out the #python channel on our Powertools for AWS Lambda Discord: Invite link

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 96.63%. Comparing base (8a0ae64) to head (71c3f65).
⚠️ Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
..._powertools/utilities/data_classes/event_source.py	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8332      +/-   ##
===========================================
- Coverage    96.63%   96.63%   -0.01%     
===========================================
  Files          296      296              
  Lines        14759    14764       +5     
  Branches      1243     1244       +1     
===========================================
+ Hits         14263    14267       +4     
  Misses         362      362              
- Partials       134      135       +1     

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[leandrodamascena]

Amazing work @stenczelt! Thanks a lot, APPROVED!

[boring-cyborg]

Awesome work, congrats on your first merged pull request and thank you for helping improve everyone's experience!