Add unit tests for RBAC

[WarLikeLaux]

Q	A
Is bugfix?	❌
New feature?	❌
Breaks BC?	❌
Fixed issues

What does this PR do?

Fixes trailing space in PhpManager::addChild() exception message, but mainly adds focused RBAC manager tests and closes uncovered branches in BaseManager, PhpManager, and DbManager. Moves 9 duplicate tests from PhpManagerTest/DbManagerTestCase into ManagerTestCase.

Coverage

File	Tests	Lines	MSI
BaseManager.php	29	59/59 (100%)	98% (63/64)
PhpManager.php	35 → 57	260/298 (87%) → 298/298 (100%)	94% (215/230) → 99% (268/273)
DbManager.php	50 → 70	416/451 (92%) → 451/451 (100%)	67% (272/405) → 69% (309/443)

Changelog not required - the only source change is a trailing space fix in an exception message in PhpManager::addChild().

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.90%. Comparing base (068c5a4) to head (c2729db).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #20785      +/-   ##
============================================
+ Coverage     78.62%   78.90%   +0.27%     
  Complexity    11566    11566              
============================================
  Files           376      376              
  Lines         30748    30748              
============================================
+ Hits          24176    24262      +86     
+ Misses         6572     6486      -86     

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