Implement tree object validation

**What**

Implement an additional level of validation to catch errors in `manifest.tree`.

**Why**

As a user, I want to be sure that the manifest I have implemented, doesn't have any structural mistakes.
Currently, the `manifest.tree` object is not validated, so it requires an additional layer of scrutiny to avoid mistakes.

**SoW (scope of work)**

Implement zod validation for checking tree object structure.

- [ ] documentation updated
- [ ] test cases added

**Acceptance criteria**

**Given** (Setup): Describes the initial state of the system or the preconditions for a change.
We have a manifest with an invalid tree structure.
**When** (Action): Describes the specific action or behavior that is being tested/changed.
Running Impact framework on the manifest with invalid tree structure.
**Then** (Assertion): Defines the expected outcome or behavior of the system after the action in the "When" step is performed.
The framework should error out.

Start at the manifest.tree validation entry point in the Impact framework and inspect how existing manifest validation is structured. Add the tree-structure validation, update the documentation, and add test cases showing that an invalid tree causes the framework to error.

Written by the indexing model from the issue text.