[Feat] Transaction 구성 및 테스트 고도화

[Zerohertz]

What type of PR is this?

• feat
• fix
• refactor
• test
• infra
• ci/cd
• docs
• chore

What this PR does / why we need it

• �Transactional한 비동기 DB 세션 ([Feat] Transaction 구성하기 #18)
• 그에 따른 PyTest 코드 개발 및 DB 연관 테스트 세션 설정 수정 ([Bug] PyTest 시 DB 연결 오류 #19)

Which issue(s) this PR fixes

Fixes #18, #19

Special notes for your reviewer

Does this PR introduce a user-facing change?

- �Transactional한 비동기 DB 세션 (#18)
- 그에 따른 PyTest 코드 개발 및 DB 연관 테스트 세션 설정 수정 (#19)

Additional documentation

[codecov]

Codecov Report

Attention: Patch coverage is 93.11475% with 21 lines in your changes missing coverage. Please review.

Project coverage is 89.66%. Comparing base (cd75b2c) to head (88acb56).

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
app/repositories/base.py	86.79%	6 Missing and 1 partial ⚠️
app/core/database.py	86.04%	3 Missing and 3 partials ⚠️
app/core/lifespan.py	71.42%	1 Missing and 1 partial ⚠️
app/exceptions/base.py	50.00%	2 Missing ⚠️
app/tests/conftest.py	91.30%	2 Missing ⚠️
app/exceptions/handlers.py	50.00%	1 Missing ⚠️
app/schemas/responses.py	66.66%	1 Missing ⚠️

@@            Coverage Diff             @@
##             main      #20      +/-   ##
==========================================
+ Coverage   82.07%   89.66%   +7.58%     
==========================================
  Files          29       30       +1     
  Lines         530      716     +186     
  Branches       33       46      +13     
==========================================
+ Hits          435      642     +207     
+ Misses         78       53      -25     
- Partials       17       21       +4     

Flag	Coverage Δ
mysql	89.24% <92.78%> (+7.54%)	⬆️
postgresql	88.96% <92.13%> (+7.26%)	⬆️
sqlite	88.54% <91.47%> (+7.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
api/v1	100.00% <ø> (+4.76%)	⬆️
core	88.63% <88.57%> (-0.35%)	⬇️
exceptions	54.34% <76.92%> (+8.40%)	⬆️
models	100.00% <ø> (ø)
repositories	90.66% <87.27%> (+33.31%)	⬆️
schemas	85.96% <91.66%> (+2.98%)	⬆️
services	88.13% <100.00%> (+13.13%)	⬆️
utils	86.76% <ø> (ø)

Files with missing lines	Coverage Δ
app/core/configs.py	89.74% <100.00%> (+2.24%)	⬆️
app/core/container.py	100.00% <100.00%> (ø)
app/core/middlewares.py	83.78% <100.00%> (+6.86%)	⬆️
app/exceptions/database.py	100.00% <100.00%> (ø)
app/main.py	100.00% <100.00%> (ø)
app/repositories/users.py	100.00% <100.00%> (ø)
app/schemas/base.py	100.00% <100.00%> (ø)
app/services/base.py	88.09% <100.00%> (+19.52%)	⬆️
app/tests/api/v1/test_shields.py	100.00% <100.00%> (ø)
app/tests/api/v1/test_users.py	100.00% <100.00%> (ø)
... and 8 more

... and 2 files with indirect coverage changes

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