Refactor web_search to reuse agent instance as function tool

[thegovind]

• The web_search() method creates a new agent every time it's invoked.
• Consider creating one instance of the agent and converting it to a function tool for reuse.

Acceptance Criteria:

• Agent is not created repeatedly in web_search().
• Function tool pattern is used for reuse.
• Tests verify the new pattern works as expected.