test: improve seata-spring-boot-starter test coverage to 96% (#6505)

[DoChaoing]

What this PR does

Improves the test coverage of seata-spring-boot-starter module to 96% instruction coverage, addressing #6505.

Changes

• pom.xml: Added spring-test dependency (test scope)
• SeataAutoConfigurationTest: 7 tests (bean creation, disabled scenario, properties defaults/setters)
• SeataDataSourceAutoConfigurationTest: 9 tests (proxy creator, disabled states, JDK/CGLIB proxy, excludes)
• SeataHttpAutoConfigurationTest: 7 tests (web/non-web context, interceptor toggle, jakarta/javax fallback)
• SeataSagaAutoConfigurationTest: 4 tests (state machine config/engine, async thread pool)

Coverage Results

Metric	Value
Instruction Coverage	96%
Method Coverage	100%
Class Coverage	100%
Total Tests	28

Closes #6505

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.78%. Comparing base (a1bb731) to head (3be1427).

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x    #7995      +/-   ##
============================================
+ Coverage     71.74%   71.78%   +0.04%     
  Complexity      872      872              
============================================
  Files          1310     1310              
  Lines         49977    49977              
  Branches       5945     5945              
============================================
+ Hits          35856    35876      +20     
+ Misses        11160    11145      -15     
+ Partials       2961     2956       -5     

see 7 files with indirect coverage changes

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