Job Monitor: extract poll completion planning and reduce mutable state

[mmitche]

Parent: #17171

Separate observation, reconciliation, completion gating, and exit-code decisions from the polling mechanism.

Scope

• Introduce an immutable poll observation/snapshot.
• Produce explicit continue/succeed/fail decisions from current observations and accumulated outcomes.
• Separate AzDO completion/failure policy from Helix current-attempt completion policy.
• Reduce MonitorState to narrowly scoped invocation bookkeeping and explicit outcome/upload ledgers.

Acceptance criteria

• Poll-loop control flow performs I/O and executes decisions rather than embedding policy.
• Completion and pass/fail rules are covered by pure tests and existing end-to-end scenarios.
• Shared mutable state is materially reduced.