fix(bigframes): escape backtick in ordering_clause identifier

[Samin061]

ordering_clause drops the column id straight into a backtick-quoted identifier without escaping, so a column name that contains a backtick closes the identifier early and the trailing text is parsed as SQL and run under the reader's BigQuery credentials. The SELECT compile path and this module's own identifier() helper already escape names with escape_chars, so the ORDER BY term now goes through the same escaping.

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

[gemini-code-assist]

Code Review

This pull request updates the ordering_clause function in bigframes/core/sql/__init__.py to escape special characters, such as backticks, in column names to prevent SQL injection or syntax errors. It also adds corresponding unit tests in tests/unit/core/test_sql.py to verify the escaping behavior. There are no review comments, so I have no feedback to provide.