feat(browser): Add viewport configuration support to BrowserClient

[sundargthb]

Description of changes:

Add viewport optional parameter to the BrowserClient.start() method to properly configure browser viewport dimensions through the underlying StartBrowserSession API.

uv-lock..................................................................Passed
ruff (legacy alias)......................................................Passed
ruff format..............................................................Passed
trim trailing whitespace.................................................Passed
fix end of files.........................................................Passed
check toml...............................................................Passed
check json...........................................(no files to check)Skipped
check for merge conflicts................................................Passed
check for added large files..............................................Passed
debug statements (python)................................................Passed
bandit...................................................................Passed
pytest with coverage.....................................................Passed