Add `packageManager` config to JS/TS hook executors

[wbreza]

Add packageManager config to JS/TS hook executors

Background & Motivation
Users running JS/TS hooks need to use the same package manager as their project (pnpm, yarn). Today the hook executor auto-detects from lock files but provides no override, unlike the framework service which supports ServiceConfig.Config["packageManager"]. This creates a parity gap between service configuration and hook configuration.

User Story
As a developer using pnpm/yarn, I want to specify packageManager in my hook config so that my JS/TS hooks use the correct package manager without relying on lock file detection.

Solution Approach

• Read Config["packageManager"] in JS/TS executor Prepare() phase via execCtx.Config
• Reuse existing packageManagerFromConfig() validation pattern from framework_service_node.go
• Pass override to node.NewCliWithPackageManager() instead of auto-detecting
• Applies to both js_executor.go and ts_executor.go (shared via node_helpers.go)
• Define a strongly-typed nodeHookConfig struct and unmarshal from execCtx.Config:

  type nodeHookConfig struct {
      PackageManager string `json:"packageManager"` // npm, pnpm, yarn
  }

• Accepted values: npm, pnpm, yarn (same as ServiceConfig)

Example azure.yaml

hooks:
  postprovision:
    run: ./hooks/setup.ts
    kind: ts
    config:
      packageManager: pnpm

Acceptance Criteria

• config.packageManager: pnpm causes JS/TS hooks to use pnpm for install
• config.packageManager: yarn causes JS/TS hooks to use yarn for install
• Omitting packageManager preserves current auto-detection behavior (lock file → default npm)
• Invalid values (e.g., bun) produce a clear error message
• Config is unmarshalled into a strongly-typed struct (not raw map access)
• Unit tests covering override, fallback, and invalid value cases

Default when omitted: Auto-detect from lock files (existing behavior)

Out of Scope

• Custom install flags (e.g., --no-audit) — future enhancement
• TypeScript runtime selection (tsRuntime) — separate P1 issue
• Bun or Deno support — not currently supported in azd

Testing Expectations

• Unit tests: table-driven tests for config unmarshalling, validation, and CLI selection
• Test that override beats auto-detection when both are present
• Test invalid/malformed config values

Constraints

• Must match the same packageManager values accepted by ServiceConfig.Config for consistency

Related Issues

• Depends on Add generic config property bag to HookConfig #7653 / PR Add executor-specific config property bag to hooks with strongly-typed schema #7690 — provides the Config map[string]any on ExecutionContext
• Parent Epic: Multi-Language Hook Support: Python, JavaScript, TypeScript, .NET #7435 — Multi-Language Hook Support