Refactor: streamline tool handling in chat.py

[SyedaAnshrahGillani]

This pull request refactors the tool-handling logic in gpt_oss/chat.py to
improve code quality, reduce redundancy, and enhance maintainability.

Previously, the code for processing browser and python tools was nearly
identical, leading to duplication. This refactor introduces a more generic
and unified approach by:

• Creating a single run_tool async function to handle different tool calls.
• Using a dictionary to map tool names to their respective tool objects.
• Replacing the repetitive if/elif block with a streamlined lookup and
  execution flow.

These changes make the code cleaner, easier to read, and simplifies the
process of adding new tools in the future.

[balajirajput96]

This pull request refactors the tool selection logic in the main function of gpt_oss/chat.py to make it more concise and maintainable. The changes simplify how browser and python tools are handled by unifying their logic into a single code path.

Tool selection and processing improvements:

• Combined the handling of browser and python tool calls by introducing a tool_map dictionary and a unified code path, reducing code duplication and improving maintainability.
• Simplified the asynchronous tool processing by using a single run_tool function for both tools.