ci(mcp-diff): pin to cross-spec-aware mcp-server-diff (3c2d5ea)

Pins both the stdio and streamable-http MCP Server Diff jobs to the 3.0
branch of SamMorrowDrums/mcp-server-diff (commit 3c2d5ea), which normalizes
cross-spec-version churn: _meta protocol plumbing, CacheableResult cache
hints, the initialize envelope, and tool-annotation default hints. Without it
the go-sdk v1.6.1 -> v1.7.0-pre.1 bump would surface ~113 spurious
idempotentHint/readOnlyHint:false diffs from the SDK dropping omitempty.

Temporary commit pin; move to the v3.0.0 tag once it ships.

Refs: github/copilot-mcp-core#1709

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Copilot

.github/workflows/mcp-diff.yml

@@ -50,7 +50,11 @@ jobs:
           } >> "$GITHUB_OUTPUT"
 
       - name: Run MCP Server Diff
-        uses: SamMorrowDrums/mcp-server-diff@v2.3.5
+        # Pinned to the 3.0 branch (cross-spec-version normalization: _meta plumbing,
+        # cache hints, initialize envelope, and tool-annotation default hints) so the
+        # go-sdk v1.6.1 -> v1.7.0-pre.1 bump produces a signal-only diff. Move to the
+        # v3.0.0 tag once it ships. See github/copilot-mcp-core#1709.
+        uses: SamMorrowDrums/mcp-server-diff@3c2d5ea
         with:
           setup_go: "false"
           install_command: |
@@ -114,7 +118,8 @@ jobs:
           } >> "$GITHUB_OUTPUT"
 
       - name: Run MCP Server Diff (streamable-http)
-        uses: SamMorrowDrums/mcp-server-diff@v2.3.5
+        # Pinned to the 3.0 branch — see rationale on the stdio job above.
+        uses: SamMorrowDrums/mcp-server-diff@3c2d5ea
         with:
           setup_go: "false"
           install_command: |