fix(browser): make Chromium sandbox configurable

[deswanth12]

Summary

Fixes #2371 by making Chromium sandbox behavior configurable and ensuring the sandbox setting is respected across browser launch paths.

Changes

• Add BrowserConfig.chromium_sandbox for explicit programmatic configuration.
• Support CRAWL4AI_CHROMIUM_SANDBOX=true as the environment-based fallback when the config is unspecified.
• Remove --no-sandbox and pass Playwright's chromium_sandbox=True when sandboxing is enabled.
• Apply the policy to regular and persistent browser launch paths.
• Keep the setting outside the untrusted request configuration allowlist.
• Add regression tests for defaults, environment fallback, explicit overrides, serialization/cloning, and untrusted configuration.

Validation

• Sandbox regression tests: 6 passed.
• Related browser and configuration tests: 36 passed.
• Ruff checks: passed.
• Git diff whitespace check: passed.

Runtime note: Linux/Docker sandbox operation still depends on compatible container and host security settings.

Fixes #2371