Add tests for deprecated tool parameter shims

[Kamilbenkirane]

Priority: Critical
Related: #199

The web_search=True → tools=[WebSearch()] migration shim (text/client.py:54-64) has zero tests. No verification that:

• The deprecation warning fires
• The shim produces the same behavior as tools=[WebSearch()]
• stacklevel=4 points to the correct caller frame
• Combining old + new params works or errors appropriately

What to fix:

• Add test asserting DeprecationWarning is emitted for web_search=True
• Add test asserting x_search=True and code_execution=True shims work
• Add test for combining web_search=True with tools=[...]
• Verify stacklevel points correctly

Files:

• src/celeste/modalities/text/client.py:43-67

[Kamilbenkirane]

Not relevant for the tools PR — deprecation shims are a follow-up concern, not a merge blocker.