Discourage agents from opening new tabs when there are others to use

[kycutler]

Feedback from @pierceboggan and @digitarald

Agents can tend to forget they have already opened a browser tab and just keep opening new ones, spamming the editor with defunct tabs.

We should try to discourage agents from opening a new tab when there is one already open and available to the agent.

[kycutler]

Verification steps:

With workbench.browser.enableChatTools: false:

1. Open an integrated browser tab to any site
2. Ask the agent to open that same site in the integrated browser
3. The tool call should result in Already open: <link to tab>
4. Ask the agent again to open it "in a second tab"
5. The page should open in a new tab (the agent passes a forceNew flag to the tool)

With workbench.browser.enableChatTools: true:

Same steps as above, but in step 2, click the "Share with Agent" button just after sending the prompt.

• This way the agent doesn't "see" the tab when the prompt is sent and won't try to pass the forceNew flag from the start.
• The deduplication logic only considers tabs that are shared with the agent.

There should be no problem opening new tabs if there is not an existing tab with the same hostname + port.

[eleanorjboyd]

it was already open (you can see it figured that out) but then it still forced the browser open)