[Backlog] Type-system follow-up: decide leaf transformation attribute rollout

[lipkau]

Problem

Issue #612 promotes JiraPS leaf return types to AtlassianPS.JiraPS.* classes and introduces JiraLeafTransformationAttribute<T> plus concrete leaf transformation attributes.

Those attributes are useful if a later parameter sweep types cmdlet parameters as leaf classes, but today most public parameters were intentionally not flipped as part of PR4.
This leaves a design question: keep the leaf transformers now as prepared surface area, remove them until a real parameter rollout needs them, or schedule a focused leaf-parameter sweep.

Proposal

Audit the promoted leaf types and public cmdlet parameters that could reasonably consume them.
For each leaf type, decide one of:

• Keep the transformation attribute because an upcoming parameter change will use it.
• Remove/defer the transformation attribute until a concrete public parameter needs it.
• Implement a small parameter sweep with docs and tests where the typed parameter is clearly valuable.

Document the decision in code/tests/docs so future type-system work does not have to rediscover the rationale.

Acceptance criteria

• Every concrete leaf transformation attribute introduced by issue [Backlog] Type-system PR4: promote 15 leaf types + tighten remaining object slots on core classes #612 is either used by a public parameter, explicitly retained with rationale, or removed/deferred.
• Any retained-but-unused attributes have unit coverage for string, typed-object, and invalid input behavior.
• If parameters are flipped to leaf types, their cmdlet help and parameter-type tests are updated in the same change.
• Invoke-Build -Task Build, Test passes.

Related items

• Parent: [Backlog] Type-system PR4: promote 15 leaf types + tighten remaining object slots on core classes #612.
• Related parameter cleanup: [Backlog] Type-system PR3: drop redundant ValidateScript blocks + cmdlet help consistency sweep #611 and [Backlog] Type-system follow-up: clean up two orphan #ToDo:CustomClass markers (Add-JiraIssueWatcher, Get-JiraIssueEditMetadata) #647.