Extract shared _map_user_tool() from duplicated ToolsMapper implementations

[Kamilbenkirane]

Priority: Critical
Related: #199

_map_user_tool() is implemented identically in both chatcompletions/parameters.py and openresponses/parameters.py. Should be extracted to a shared base class or utility.

What to fix:

• Extract _map_user_tool() to a shared ToolsMapper base in celeste.tools or celeste.parameters
• Have both protocol ToolsMappers inherit from or call the shared implementation

Files:

• src/celeste/protocols/chatcompletions/parameters.py:112-129
• src/celeste/protocols/openresponses/parameters.py:165-182

[Kamilbenkirane]

Downgraded: code duplication across wire formats is a refactoring task, not a merge requirement. The 4 implementations are correct and intentionally different. Closing — can revisit post-merge if desired.

[Kamilbenkirane]

Closing — pure duplication removal for working code, not worth the indirection cost right now.