[ty] Add workflow to comment conformance tests diff (#19555)

## Summary

Follow-up to https://github.com/astral-sh/ruff/pull/19556, this PR adds
the workflow that computes the diagnostic diff which the workflow
introduced in the linked PR will add as a comment.

This workflow is similar to the [ty ecosystem-analyzer
workflow](d781a6ab3f/.github/workflows/ty-ecosystem-analyzer.yaml).

Closes: astral-sh/ty#212

## Test Plan

1. Initially there's no diff to show
2. This
[commit](d0db9937df)
comments out a rule which updates the comment with the diff
3. Later, that commit is reverted and the diff goes away

Use the comment history to look at the diff output where the order of
the history corresponds to the steps mentioned above in reverse order
i.e., the edit in the middle will contain the diff output:

<img width="1082" height="313" alt="Screenshot 2025-07-25 at 21 09 26"
src="https://github.com/user-attachments/assets/6aceb60c-1987-4b9a-9063-e3999844f035"
/>

.github/workflows/typing_conformance.yaml

@@ -0,0 +1,109 @@
+name: Run typing conformance
+
+permissions: {}
+
+on:
+  pull_request:
+    paths:
+      - "crates/ty*/**"
+      - "crates/ruff_db"
+      - "crates/ruff_python_ast"
+      - "crates/ruff_python_parser"
+      - ".github/workflows/typing_conformance.yaml"
+      - ".github/workflows/typing_conformance_comment.yaml"
+      - "Cargo.lock"
+      - "!**.md"
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref_name }}-${{ github.event.pull_request.number || github.sha }}
+  cancel-in-progress: true
+
+env:
+  CARGO_INCREMENTAL: 0
+  CARGO_NET_RETRY: 10
+  CARGO_TERM_COLOR: always
+  RUSTUP_MAX_RETRIES: 10
+  RUST_BACKTRACE: 1
+
+jobs:
+  typing_conformance:
+    name: Compute diagnostic diff
+    runs-on: depot-ubuntu-22.04-32
+    timeout-minutes: 10
+    steps:
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+        with:
+          path: ruff
+          fetch-depth: 0
+          persist-credentials: false
+
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+        with:
+          repository: python/typing
+          ref: d4f39b27a4a47aac8b6d4019e1b0b5b3156fabdc
+          path: typing
+          persist-credentials: false
+
+      - name: Install the latest version of uv
+        uses: astral-sh/setup-uv@bd01e18f51369d5a26f1651c3cb451d3417e3bba # v6.3.1
+
+      - uses: Swatinem/rust-cache@98c8021b550208e191a6a3145459bfc9fb29c4c0 # v2.8.0
+        with:
+          workspaces: "ruff"
+
+      - name: Install Rust toolchain
+        run: rustup show
+
+      - name: Compute diagnostic diff
+        shell: bash
+        run: |
+          RUFF_DIR="$GITHUB_WORKSPACE/ruff"
+
+          # Build the executable for the old and new commit
+          (
+            cd ruff
+
+            echo "new commit"
+            git checkout -b new_commit "${{ github.event.pull_request.head.sha }}"
+            git rev-list --format=%s --max-count=1 new_commit
+            cargo build --release --bin ty
+            mv target/release/ty ty-new
+
+            echo "old commit (merge base)"
+            MERGE_BASE="$(git merge-base "$GITHUB_SHA" "origin/$GITHUB_BASE_REF")"
+            git checkout -b old_commit "$MERGE_BASE"
+            git rev-list --format=%s --max-count=1 old_commit
+            cargo build --release --bin ty
+            mv target/release/ty ty-old
+          )
+
+          (
+            cd typing/conformance/tests
+
+            echo "Running ty on old commit (merge base)"
+            "$RUFF_DIR/ty-old" check --color=never --output-format=concise . > "$GITHUB_WORKSPACE/old-output.txt" 2>&1 || true
+
+            echo "Running ty on new commit"
+            "$RUFF_DIR/ty-new" check --color=never --output-format=concise . > "$GITHUB_WORKSPACE/new-output.txt" 2>&1 || true
+          )
+
+          if ! diff -u old-output.txt new-output.txt > typing_conformance_diagnostics.diff; then
+            echo "Differences found between base and PR"
+          else
+            echo "No differences found"
+            touch typing_conformance_diagnostics.diff
+          fi
+
+          echo ${{ github.event.number }} > pr-number
+
+      - name: Upload diff
+        uses: actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02 # v4.6.2
+        with:
+          name: typing_conformance_diagnostics_diff
+          path: typing_conformance_diagnostics.diff
+
+      - name: Upload pr-number
+        uses: actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02 # v4.6.2
+        with:
+          name: pr-number
+          path: pr-number