fix: correct "Decomissioned" typo to "Decommissioned" in safekeeper status

[MaxwellCalkin]

Note: This PR was authored by Claude (AI), operated by @MaxwellCalkin.

Fixes #12832

Summary

The SkSchedulingPolicy enum variant was misspelled as Decomissioned (missing one "m") instead of Decommissioned. This typo propagated to API responses, string serialization, comments, documentation, and tests across 7 files.

Changes

Rust source (libs/pageserver_api/src/controller_api.rs)

• Rename enum variant Decomissioned to Decommissioned
• Add #[serde(alias = "Decomissioned")] for backwards-compatible JSON deserialization
• Accept both "decommissioned" and "decomissioned" in FromStr for database compatibility
• Update canonical From<SkSchedulingPolicy> for String output to "decommissioned"

Storage controller (storage_controller/src/)

• Update all SkSchedulingPolicy::Decomissioned references to SkSchedulingPolicy::Decommissioned in:
  • heartbeater.rs
  • service/safekeeper_service.rs
  • service/safekeeper_reconciler.rs
• Fix misspelled comments ("decomissioned" -> "decommissioned")

Database migration

• Add migration 2026-03-08-000001_fix_decommissioned_typo to update existing scheduling_policy = 'decomissioned' rows to 'decommissioned'

Documentation

• Fix typo in docs/rfcs/035-safekeeper-dynamic-membership-change.md

Tests

• Update string literals in test_runner/regress/test_safekeeper_migration.py and test_runner/regress/test_storage_controller.py

Backwards compatibility

• JSON deserialization: The #[serde(alias = "Decomissioned")] attribute ensures that JSON payloads using the old spelling are still accepted
• Database: The FromStr implementation accepts both "decomissioned" and "decommissioned", and the migration updates existing rows
• API responses: Will now return the correctly-spelled "Decommissioned" (via serde) and "decommissioned" (via From<> for String)