Add search engine feature

[adityasoni9998]

This PR adds a search engine feature to OpenHands.

Changes:

1. Add search engine tool to codeact agent
2. Add search action and observation types
3. Add tests for search engine feature

Tests:

• Added test_get_tools_with_options to verify search engine tool is enabled/disabled correctly
• Added test_search_engine_tool to verify tool schema
• Added test_response_to_actions_search_engine to verify action creation from tool calls

Fix: #7262

[neubig]

@OpenHands please do the following:

1. check the diff with the base branch and all the PR comments to fully understand what is going on here
2. reflect all review comments and also solve all merge conflicts with main
3. double check that the PR is limited to only relevant changes, and doesn't make other extraneous changes
4. reflect all the review comments
5. run each of the steps in the py-unit-tests.yml workflow to make sure that tests are passing

[openhands-ai]

Openhands is working, @neubig can track my progress at all-hands.dev

[neubig]

@OpenHands add user-facing documentation of this feature. It should be included in the "Advanced Configuration" section of the docs. Please view other pages in that section and understand the structure and content of other similar docs, then add some docs for this feature. You can keep it concise, no need to be overly wordy.

[openhands-ai]

Openhands is working, @neubig can track my progress at all-hands.dev

[BradKML]

What is the current progress with this and search engines (e.g. SearXNG or Startpage or Qwant)?

[xingyaoww]

@BradKML I think this PR itself is working.. The reason we haven't merge this is that this adds yet another potential env var we need to worry about (i.e., SEARCH ENGINE API KEY) that may complicate the user experience (user need to input yet another key to use OpenHands) OR the developer experience (if we want to automatically turn off search IF user didn't provide the key).

I'm thinking if this can be taken care of by the MCP (#7637) that will give us search engine & other things, and we can just implement these "selective turn off when no API key is provided" in one place