feat: add SessionConfiguration with proxy, extensions, and profile support

[kevin-orellana]

Summary

• Add composable dataclasses for browser session configuration in config.py, following the established BrowserConfiguration pattern
• ProxyCredentials, ExternalProxy, ProxyConfiguration for proxy routing
• ExtensionS3Location, BrowserExtension for loading browser extensions
• ProfileConfiguration for persisting browser state across sessions
• SessionConfiguration composite that bundles viewport, proxy, extensions, and profile into start() kwargs
• Add extensions and profile_configuration parameters to start() and browser_session()

Usage: client.start(**session_config.to_dict())

Test plan

• All existing proxy tests still pass (plain dict passthrough)
• New unit tests for all config dataclasses and browser_client parameters (96 unit tests pass)
• 23/23 integration tests pass against live API (python3 tests_integ/tools/test_browser_proxy.py):
  • Proxy via plain dict and via ProxyConfiguration dataclass
  • SessionConfiguration with proxy, viewport, extensions, profile individually and combined
  • browser_session() context manager driven by SessionConfiguration
  • Failure modes: double stop() idempotency, context manager cleanup on exception, get_session() after stop
  • Error handling: invalid secret ARN, unreachable proxy server, malformed proxy config, multiple extensions

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

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  Misses          ?      170           
  Partials        ?      145           

Flag	Coverage Δ
unittests	91.20% <100.00%> (?)

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[stevegmf]

LGTM