refactor: make cluster singleton spawning more resilient

[Tochemey]

• Refactors SpawnSingleton to be idempotent across leader churn and name conflicts, avoiding false errors when the singleton is already registered.
• Centralises and simplifies retry behaviour for transient cluster/remoting conditions, improving resiliency without changing public APIs.
• Adds clearer error handling paths for singleton creation, including role-based placement and leader forwarding.

closes #1036

[TrueBrain]

All these additions are awesome. I really like the new options! And that I can run SpawnSingleton in an idempotent way makes reasoning about singletons a lot easier (for me at least) :)

One question: the new options, do they also apply during rebalancing? As that still fails with the default partition count of 271 (it works when set to 9) for me.
Edit: I can fully understand if they don't apply to rebalance btw; mostly checking if I am understanding it correct :)

[Tochemey]

All these additions are awesome. I really like the new options! And that I can run SpawnSingleton in an idempotent way makes reasoning about singletons a lot easier (for me at least) :)

One question: the new options, do they also apply during rebalancing? As that still fails with the default partition count of 271 (it works when set to 9) for me. Edit: I can fully understand if they don't apply to rebalance btw; mostly checking if I am understanding it correct :)

can we please chat on the issue rather than on the PR?

[codecov]

Codecov Report

❌ Patch coverage is 93.38843% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.13%. Comparing base (15ed35e) to head (76e58df).

Files with missing lines	Patch %	Lines
actor/pid.go	28.57%	8 Missing and 2 partials ⚠️
actor/spawn.go	95.45%	2 Missing and 2 partials ⚠️
actor/actor_system.go	93.10%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1038      +/-   ##
==========================================
+ Coverage   93.95%   94.13%   +0.17%     
==========================================
  Files         144      146       +2     
  Lines       11862    12053     +191     
==========================================
+ Hits        11145    11346     +201     
+ Misses        468      459       -9     
+ Partials      249      248       -1     

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