[TypeSpec Authoring] Integrate TypeSpec breaking-change tool into assessment

## Parent issue

Part of #14529.

## Goal

Integrate the TypeSpec team's breaking-change tool led by Mark into the assessment evidence and decision flow.

## Scope

- Confirm the tool's supported inputs, outputs, versioning, availability, failure modes, and ownership with Mark.
- Define an adapter that normalizes tool findings into the assessment result contract.
- Preserve raw tool evidence and source locations for traceability.
- Reconcile tool findings with existing REST and downstream assessment logic without duplicate or contradictory findings.
- Define behavior when the tool is unavailable, fails, or returns incomplete evidence.

## Acceptance criteria

- Assessment can invoke a supported version of the tool and render normalized findings with source evidence.
- Duplicate findings are consolidated while disagreements remain explicit and reviewable.
- Tool failures produce a clear incomplete-analysis status rather than a safe result.
- Tests cover successful results, no findings, disagreement, malformed output, and tool failure.

Start with the assessment evidence and decision flow in the repository, then confirm the breaking-change tool’s inputs, outputs, versioning, availability, failure modes, and ownership with Mark. Define the adapter around the assessment result contract and preserve raw evidence and source locations. Done means normalized findings, explicit disagreements, incomplete-analysis status on failures, and tests for success, no findings, malformed output, disagreement, and tool failure.

Written by the indexing model from the issue text.