Add coverage baseline commit to git-blame-ignore

Adds commit 89e9c43 to
.git-blame-ignore-revs to exclude the large-scale addition of
# pragma: no cover comments from git blame output.

This commit established a 100% code coverage baseline by adding pragma
comments throughout the codebase. While important for coverage tracking,
these annotations are noise in git blame and should be ignored to show
the original authors of substantive code changes.

Github-Issue: #1553

Author: maxisbey

.git-blame-ignore-revs

@@ -1,2 +1,5 @@
 # Applied 120 line-length rule to all files: https://github.com/modelcontextprotocol/python-sdk/pull/856
 543961968c0634e93d919d509cce23a1d6a56c21
+
+# Added 100% code coverage baseline with pragma comments: https://github.com/modelcontextprotocol/python-sdk/pull/1553
+89e9c43acf7e23cf766357d776ec1ce63ac2c58e