Add shared config unmarshalling helper for hook executors

Background & Motivation
Each hook executor will need to unmarshal execCtx.Config (a map[string]any) into a strongly-typed struct. Rather than duplicating unmarshalling logic across every executor, we should provide a shared helper that handles the conversion, nil-safety, and error formatting consistently.

User Story
As a hook executor developer, I want a shared helper to unmarshal the Config property bag into a typed struct so that I don't duplicate boilerplate across executors.

Solution Approach

• Create a generic helper function in pkg/tools/ (or pkg/ext/):

  // UnmarshalHookConfig unmarshals the raw config map into a typed struct.
  // Returns a zero-value T if config is nil or empty.
  func UnmarshalHookConfig[T any](config map[string]any) (T, error)

• Implementation: re-marshal map[string]any → JSON → unmarshal into T (same pattern used by ai.ParseConfig and UnmarshalStruct elsewhere in the codebase)
• Handle nil/empty config gracefully (return zero-value struct, no error)
• Provide clear error messages on type mismatches
• Each executor calls this in Prepare():

  cfg, err := tools.UnmarshalHookConfig[nodeHookConfig](execCtx.Config)
  if err != nil {
      return fmt.Errorf("invalid hook config for JS executor: %w", err)
  }

Example Usage in Executors

// In js_executor.go Prepare():
type nodeHookConfig struct {
    PackageManager string `json:"packageManager"`
}

cfg, err := tools.UnmarshalHookConfig[nodeHookConfig](execCtx.Config)
if err != nil {
    return fmt.Errorf("invalid JS hook config: %w", err)
}
// cfg.PackageManager is now typed and ready to use

Acceptance Criteria

• Generic helper function UnmarshalHookConfig[T] exists and is tested
• Nil/empty config returns zero-value struct with no error
• Invalid types produce clear error messages (e.g., packageManager: 123 when string expected)
• Works with all executor config structs (nodeHookConfig, pythonHookConfig, dotnetHookConfig)
• Unit tests covering nil config, empty config, valid config, and type mismatch cases
• Follows existing patterns (ai.ParseConfig, UnmarshalStruct)

Default when omitted: N/A — this is a helper, not a user-facing config

Out of Scope

• Config validation beyond type checking (each executor validates its own values)
• Schema generation from config structs

Testing Expectations

• Unit tests: table-driven tests with various config shapes
• Test nil safety, empty map safety, type mismatch errors
• Test with representative structs from each executor

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
• Blocks: JS/TS, Python, .NET executor config issues (provides the shared helper they consume)
• Parent Epic: Multi-Language Hook Support: Python, JavaScript, TypeScript, .NET #7435 — Multi-Language Hook Support