fix: honor Option contract on tryGetExistingExecution miss

[Ma77Ball]

What changes were proposed in this PR?

• Change tryGetExistingExecution in ExecutionsMetadataPersistService to wrap the DAO call with Option(...) instead of Some(...), so a missing row (jOOQ returns null) collapses to None rather than Some(null).
• Keep the existing catch Throwable block, which still earns its keep for hard DB errors (e.g. closed connection); the Option wrap only addresses the no-row miss path.
• Update ExecutionsMetadataPersistServiceSpec (added in test(amber): DB-backed unit tests for ExecutionsMetadataPersistService and updateWorkflowState #5213): replace the two paired pin tests (the Some(null) pin and the inverted intercept[TestFailedException] xfail-strict) with a single positive shouldBe None test.

Any related issues, documentation, or discussions?

Closes: #5211. Depends on #5213 (test scaffolding); rebase after it lands.

How was this PR tested?

• Ran sbt scalafmtAll (no rewrites needed).
• ExecutionsMetadataPersistServiceSpec.tryGetExistingExecution now has positive tests for both the hit (Some(row)) and the miss (None) cases.

Was this PR authored or co-authored using generative AI tooling?

Co-authored with Claude Opus 4.7 in compliance with ASF

[Ma77Ball]

/request-review @aglinxinyuan

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 48.54%. Comparing base (24f7702) to head (9079dbe).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #5221      +/-   ##
============================================
- Coverage     48.55%   48.54%   -0.02%     
+ Complexity     2372     2370       -2     
============================================
  Files          1042     1042              
  Lines         39971    39971              
  Branches       4252     4252              
============================================
- Hits          19408    19403       -5     
- Misses        19414    19418       +4     
- Partials       1149     1150       +1     

Flag	Coverage Δ		*Carryforward flag
access-control-service	39.53% <ø> (ø)		Carriedforward from 24f7702
agent-service	33.76% <ø> (ø)		Carriedforward from 24f7702
amber	51.48% <100.00%> (-0.04%)	⬇️
computing-unit-managing-service	0.00% <ø> (ø)		Carriedforward from 24f7702
config-service	0.00% <ø> (ø)		Carriedforward from 24f7702
file-service	37.99% <ø> (ø)		Carriedforward from 24f7702
frontend	40.02% <ø> (ø)		Carriedforward from 24f7702
python	90.51% <ø> (ø)		Carriedforward from 24f7702
workflow-compiling-service	56.81% <ø> (ø)		Carriedforward from 24f7702

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