refactor(amber): rename remaining Akka* identifiers to Pekko*

[Yicong-Huang]

What changes were proposed in this PR?

The project moved off Akka onto Apache Pekko, but several internal Scala identifiers still carried the Akka prefix even though they wrap Pekko APIs. Pure rename across amber and common/config:

• AkkaConfig → PekkoConfig (object + file)
• AkkaActorService → PekkoActorService (class + file)
• AkkaActorRefMappingService → PekkoActorRefMappingService
• AkkaMessageTransferService → PekkoMessageTransferService
• akkaConfig, akkaActorService method/parameter names → pekko-prefixed

No behavior change. No string literals, config keys, or serialization registrations are touched — cluster.conf already uses pekko.* keys, the kryo registry doesn't reference these classes by name. The intentional "akka" literal in DeployStrategiesSpec.scala that contrasts pekko vs akka address strings stays.

Any related issues, documentation, discussions?

Closes #4948.

How was this PR tested?

sbt WorkflowExecutionService/Test/compile clean, sbt WorkflowExecutionService/scalafmtCheckAll clean.

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

Generated-by: Claude Opus 4.7 (Claude Code)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 82.60870% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 42.22%. Comparing base (43f276e) to head (31bf1e0).

Files with missing lines	Patch %	Lines
...ache/texera/amber/engine/common/AmberRuntime.scala	50.00%	2 Missing ⚠️
...hitecture/common/PekkoActorRefMappingService.scala	0.00%	0 Missing and 1 partial ⚠️
...hitecture/common/PekkoMessageTransferService.scala	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #4949   +/-   ##
=========================================
  Coverage     42.22%   42.22%           
  Complexity     2181     2181           
=========================================
  Files           980      980           
  Lines         36287    36287           
  Branches       3783     3783           
=========================================
+ Hits          15322    15323    +1     
  Misses        20037    20037           
+ Partials        928      927    -1     

Flag	Coverage Δ
access-control-service	39.53% <ø> (ø)
amber	43.14% <82.60%> (+<0.01%)	⬆️
computing-unit-managing-service	0.00% <ø> (ø)
config-service	0.00% <ø> (ø)
file-service	33.24% <ø> (ø)
workflow-compiling-service	47.72% <ø> (ø)

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[Yicong-Huang]

Keeping this PR rename-only on purpose so it stays trivially revertable. The 82.6% patch coverage flag is a codecov artifact: project coverage is unchanged (+1 hit, −1 partial) — those four lines were already uncovered on main, the rename just made codecov re-attribute them to this patch. Will add tests for the actor-service / AmberRuntime gap in a follow-up PR.