feat: add test 6.1.57, 58

[peinjoh]

Resolves #486 #485
This PR:

• adds a new recursive method on the product tree to depth-first visit all leaf nodes with backtracking to collect their paths
• adds test 6.1.57 and 6.1.58
• adds display for the "merged" branch categories on the trait (used for debugging during development, but is also required on the lib api, so I'm leaving it in)

Their is some functional overlap between visit_all_branches and collect_all_leaves, and this could be generalized into a generic tree visitor with a pre-visit and post-visit callback. We should consider that if more similar functionality of the product tree becomes necessary, but IMO it would be overengineered for now.

[Copilot]

Pull request overview

Adds CSAF 2.1 validator implementations for test cases 6.1.57 and 6.1.58 by introducing a reusable product-tree traversal helper that enumerates all root-to-leaf paths, and wiring the new validations into the validations module.

Changes:

• Added ProductTreeTrait::collect_leaf_paths() (and recursive helper on BranchTrait) to depth-first collect root→leaf branch paths plus JSON instance paths.
• Implemented validations for 6.1.57 (stacked branch categories) and 6.1.58 (product_version + product_version_range in same path), including unit tests.
• Extended CategoryOfTheBranch with Hash derivation and a Display impl to support hashing and readable error messages.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 7 comments.

File	Description
csaf-rs/src/validations/test_6_1_57.rs	New validation for stacked branch categories using collected leaf paths + associated tests.
csaf-rs/src/validations/test_6_1_58.rs	New validation preventing product_version and product_version_range in the same path + associated tests.
csaf-rs/src/validations/mod.rs	Exposes the new validation modules.
csaf-rs/src/csaf_traits.rs	Adds leaf-path collection traversal, plus Hash/Display on CategoryOfTheBranch.

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[peinjoh]

Abandoned after merge fail. Continued on #508