fix(mcp): close isolated contexts on client disposal

[oleksii-tumanov]

Summary

Close the browser context owned by an isolated HTTP client when it disconnects.

Regression coverage for two concurrent isolated clients:
after one disconnects, the remaining client sees one live context instead of two.

Fixes #41539

[pavelfeldman]

Somehow my agent did it with a smaller diff. Identical behavior though: https://github.com/microsoft/playwright/pull/41560/changes

[oleksii-tumanov]

Thanks @pavelfeldman . The smaller production diff make sense.

I analized both patches. I see that #41560 keeps the last-client browser shutdown path unchanged and reuses lifecycle log assertions additionally for concurrent HTTP and SSE. While old #41540 added a direct contexts().length check for direct validation.

For future contributions, would you have preferred that I revise #41540 in that direction? I would have been happy to update it based on review feedback. Understanding that would help me better match the project’s preferred workflow.