refactor(adf): replace patcher switch branches with operation handlers

[SmartBrandStrategies]

Linked feedback: ADX-007

Problem

patcher.ts relies on a large operation switch with repeated content-type checks and
index bounds logic in multiple branches, increasing maintenance overhead and error risk.

Evidence:

• Main switch: packages/adf/src/patcher.ts:22
• Repeated guards in add/replace/remove bullet paths: packages/adf/src/patcher.ts:49

Scope

• Introduce handler map keyed by operation (PatchOperation['op']).
• Extract shared helpers for:
  • section lookup
  • index bounds validation
  • list/map entry parsing/coercion
• Keep immutable semantics and current error behavior.

Acceptance Criteria

• Behavior parity across all patch operations and failure paths.
• Duplicated guard logic is reduced materially in patcher implementation.
• Existing patcher tests pass; new helper-focused edge-case tests added.

Test Plan

• pnpm run test --filter @stackbilt/adf
• Add tests for:
  • invalid index errors (list/map)
  • unsupported content-type errors
  • metric update on missing metric key