Improve API baseline diff output

[AndriySvyryd]

Improvements to ApiChief's diff generation and the workflow that posts API review comments on merged PRs:

Workflow base SHA: pull_request_target.base.sha is the PR's original base, so any commits pushed to the target branch while the PR was open were being included in the diff. We now use the merge commit's first parent, which is the target tip immediately before the merge — the resulting diff reflects only what the PR actually introduced.
Stage tags in diff: Types and members with a non-Stable stage now render with an inline [Experimental] / [Obsolete] tag, even when the stage itself didn't change. Previously the stage was only shown when it transitioned.
Header-only type changes: Adding/removing an interface (e.g. on RelationalMapToJsonConvention) used to produce a remove-the-whole-type-and-add-it-back diff because type identity was the full declaration string. Type matching is now based on a stable Identity (kind + name + generic arity), and the previous declaration is carried through as a single - old / + new header pair.
Word-wrapping at 160 chars: Long lines (mostly type headers with many interfaces, or methods with many parameters) are wrapped at comma/space boundaries with a 4-space continuation indent. The diff prefix (+/-/ ) is preserved on continuation lines so coloring stays correct.

[Copilot]

Pull request overview

Improves ApiChief's diff output quality and fixes the API review workflow's diff scope so posted comments reflect only what each PR actually introduced.

Changes:

• Use the merge commit's first parent as the diff base (instead of pull_request_target.base.sha) so commits landed on the target branch while the PR was open aren't included.
• Match types by a stable Identity (kind + name + generic arity, stripping base/interface list and constraints) so header-only changes (e.g., adding an interface) render as a single -old/+new header pair rather than full type removal+re-add.
• Render non-Stable stage tags inline ([Experimental]/[Obsolete]) on every type/member and wrap long diff lines at 160 chars on comma/space boundaries with a 4-space continuation indent, preserving the +/-/ prefix.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
.github/workflows/api-review-baselines.yml	Resolves base SHA to merge commit's first parent for merged PRs to scope the diff to just this PR.
eng/Tools/ApiChief/Model/ApiType.cs	Adds Identity (header-stripped) used for equality/hashing and PreviousType to carry the old header for in-memory delta.
eng/Tools/ApiChief/Model/ApiModel.cs	Detects header-only type changes and stores baseline header in PreviousType so they appear as modifications.
eng/Tools/ApiChief/Commands/EmitDelta.cs	Always emits stage tag, renders header-only changes as -old/+new, and wraps long diff lines at 160 chars preserving the diff prefix.